I have always gone for a medium frame when buying bikes in the past and been ok, but I am now looking to buy another 29er frame, but when I look at the sizing guidelines for the frame I am looking at they say for 5’11” I should go large.

Do most people go on what the company recommends of always either go large or medium and then adjust crank length, stem, swept backed seat post to tailor to their size?

i go in inchs/cm,

measure your medium and compare it to their geometry. Not all mediums are the same size.

I tend to go with what the company recommends. Sometimes that’ll be a small, sometimes it’ll be a large. It’s not even consistent within brands. Checking out Top Tube length is a good way of validating it.

The only reliable way is to sit on the bike.
End of story. 🙂

I’m just over 6′ and general fall on an 18″ or 19″ frame.

Generally it puts me between a medium and a large. I know a 19″ is just about ideal and a 20″ feels slightly too big with a straight top tube (top tube length dependant of course).

More to it than just seat tube length. Is there any way you can try one first?

This is one area where your LBS does come in handy, but it is still difficult to swallow when they want £1600 for the frame and you know if you shop around on ebay you can get it for £800.

So, if I measure my existing frame is it the top tube length that is the key one as that is exactly the same length as the frame I am looking at?

More or less. Things like Head Tube and Seat Tube angle will have an effect, which is why it’s useful to listen to what the manufacturers have to say. For instance, I’d have opted for a Small 9Zero7 frame based on the TT of my other bikes but I was recommended a Medium and am glad I listened to them. The Small would have been far too cramped.