I use small mtb frames (15 or 16"). Anyone now what size road frame would be right? I'm 5'8" and do prefer 'small' frames. cheers

No way of telling unless you go and try some.Road frames vary even more than mtbs.I`m 6ft,and my Trek is a 58,I fit on a 56 on Specialized,and 54cm on others.

Go to your LBS and ask their advice. Its the easiest simplest way of finding out.

Funnily enough this will be the first thing I cover in this: http://www.singletrackworld.com/blogs/2009/09/road-is-all-the-rage/

Expect answers next week. And hopefully a video.

Gosh Sim, that looks really interesting. Er, not.
What's up, run out of mountain bike stuff to write about?
Leave the road stuff to Cycling Weakly. (see what I did there?)

DezB, if it's not interesting to you, don't look at it.
Easy.

I was actually looking for a rough idea for when looking on ebay etc.
What size road frames are other people riding who ride small mtb frames?

The answer is still go to your LBS and ask their advice as road bike sizing is more critical than mtb sizing.
However, I'm sure that someone will come along and post some Internet based sizing system for you.

There's a website that lets you put in measurements to decide.

It's the effective toptube measurement that you need to work out. IT get's more complicated as there are different styles of road frame (compact, standard, somewhere in between, sportive etc.)

I'd personally go to a bike shop unless you were very confident of doing it from numbers.

Can't think of the website, I think it's in my favourites at home. I'll let you know. Somethink like procyclist?

I'm 5'8" and ride small frames. I prefer that always have, I size it up with the post and stem.
All my recent bikes have been compact small i.e Planet X and Giant, everything else has been 52cm C-T.
Though I do like mine saddle high bars low.
I did have a bike that was supposed to be my size a few years back, a medium Giant. It was too big, I had to run a little 100mm stem it was horrid