I've just tried a 56cm, and while it felt great to ride, I couldn't shake the feeling that I should be on a 58. I think this is based on my road bike, but maybe a CX bike should be a little bit shorter? Should I be able to see the front hub when I'm on the drops?
I would go with my initial impression but last time I bought a bike based on first impression I got it wrong...
Any advice appreciated!
I'm 5'8" and currently testing a 56cm Croix de Fer, it feels a little big and I reckon I'd be happier on a 54cm. With that in mind I would have actually said 56cm for your height.
Can you try a 58cm?
I go for 2cm smaller than my road bike as a good rule of thumb.
Gary, thanks, I'm trying a 58cm next week, but for short test rides on the pavement the smaller bike is probably always going to feel better, hence buying too small a racer first time round.
However aP's advice would suggest the 56cm after all (i ride a 58cm CAAD9).
Perhaps I'm just in between, maybe err on the larger size so at least I can get the handlebars at the right height, then tweak backwards and forwards as needs be..
I'm 5' 10" and ride a 56 Genesis Vapour. TBH I should have bought the 54 as it feels slightly too big. But then again I have short legs and knuckles that drag on the floor YMMV 
5'7" here and ride a 54" flyer. It would be fine for Proper geared road riding but as a commute bike I'd prefer it shorter. 100mm stem may get reduced.
What are you buying it for? Commuting or CX?
Just watch the amount of toe overlap on a smaller frame, not an issue when using road tyres, but a pair of knobblies and some tight switchback could see you over the bars.
Doug
I'm 5'8" and ride 56.5 road bikes and 54 cx bikes. Go with what seems right with you rather than anyone else.
Alternatively go to your LBS and ask their opinion...
I've just been looking at the Genesis geometry charts, and just to confuse even more, this particular 5'11 rider would go for a 54cm.
My reasoning being- lower headtube, slightly more weight over the front wheel, able to get a good efficient position without being too stretched...
West Kipper, I see your logic, but it was too low + short a position that caused me problems previously, too hunched and pulling my shoulders forwards too much.
aP, I'm buying it in my LBS and gpt plenty of chat with the staff there (damn right, I worked with them for three years!) but was just looking to see if there was a consensus, because there's not much in it! Will probably err on the large side for the sake of opening my upper body up a wee bit.
It really depends on how you are going to ride it. If you are going to take it off road (I'm not talking about forestry tracks), then standover may be an issue. Stems and lay back posts can be used to tweak position on a smaller bike, but if you aint got the standover clearance with a larger frame then you may come a cropper.
Oh aye, 13th, I wasn't suggesting what you should do, just adding another opinion as to what I would (and did, since my current CX is similar geometry)
Are you sure that the longer Toptube on the 58 won't stetch you out too much though? I'd definitely want to get an extended test before you stick your Groats down.
I'm 5'10" riding a 54, normally ride a 56cm road bike. check the sizing carefully - the TT doesn't increase as much from size to size as you'd think. I've got short legs and a long body so given the marginal differences in TT and the traditional geometry I went 54 for the standover.
Good advice all, with any luck I'll be able to try them side by side.
its mainly for commuting but I had hoped to throw in some longer off road commutes over corstorphine hill/through hunter's bog etc.
Looking at the geometry charts (which I probably should have done first) there's only 1cm in difference between effective top tubes, but 1.5cm difference between headtubes, which would suggest the larger size would be best assuming standover is OK.
Good. Decided. 
13thFM, when you getting it? I'm really tempted by one myself. Hope you're getting a good deal!
Next couple of weeks, but I'll be on holiday so probably won't get the wheels dirty until June.
Want a shot?
Might take you up on that!
i'm 6foot and i've got a 56cm one for commuting. seems fine for me although there is some foot/front tyre overlap (nothing serious yet!). never thought i should have got a bigger one, but saying that it came with a super short seatpost which i've replaced to get another inch out of it which makes me think it may be aimed at people a tad shorter...
I'm 5'10" and had the same questions. There's only 0.5 cm difference between the TT length for each size, (like ianpv said) and FWIW I went for the 54cm on the advice of the shop. Straight handlebar though, so the extra width brings you down and forward.
I've not looked at the geometry but 58cm sounds big. I wouldn't worry overly much about toe overlap. I have it (on a different bike) and you just ride through it, it's very rarely a problem on- or off-road in tight situations.
I'm 6'2" and have a 60cm, they feel short to me as the bars are very short reach, but this gives a nice comfortable reach to the drops and a good commanding view in traffic. I always look at the TT dimension as well as seat tube and standover.
56 would be my guess (5'9ish here and 54cm)
5ft 11 for me on a 54cm frame,
120mm stem and lay back post,feels just right.
2inch toe overlap though but only been caught on the tightest of turns.
(In general, please note) I always thought crossbike sizing was a bit like Colnago sizing; Due to the more relaxed headtube angle you go for a smaller size, and use a longer stem/ more set back seatpost to centre you over the bike. The slack head angle also means that the stem has more rise than on a 73 degree road bike, and you have a longer fork too, so a smaller headtube is also desirable.
Obviously the biggest sales of CX bikes now, seem to be to MTBers wanting something familiar to ride on the road so they seem to be spec'ed with fairly short upright bar/stems that make it easier to make the mistake of going for a too big size.
Thats interesting W.K. I'm aiming for something inbetween my mountainbikes and my road bike.
If there is a consensus above it seems to be leaning towards the 56cm, despite my originally leaning towards the 58. Still going to give it a try but will keep the above in mind!
13thfloormonk you are more than welcome to take my 56cm Vapour (same geometry I think) for an extended test ride, if you fancy a jolly up to Perthshire.
There's a 56 demo bike in EBC, had a sit on it a couple of days ago and it felt nice, i'm 6ft and would have steered towards a 58 but i'm not so sure now.
I'm 6'1" and ride a 58cm Bianchi. Tried 56, 58 and 60 in Genesis Day One frames and the 58cm is the one i will be buying.
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