What size bike?

[Slowroad]

I expect that this has already been covered but my searching on the forum didn't turn up anything (probably my incompetence).
I'm thinking about buying/building a bike for commuting and touring. It's 20 years since I bought a bike, apart from my £40 hack bike. Bike design, as well as much else, has changed just a bit in that time! I'm confused about sizing, particularly with the new compact frames, so some pointers to where to get info will be helpful.
Cheers!

[stewartpratt]

Sit on some, preferably have a go on some. It's the only way.

Take note of seat angles and (effective, ie horizontal) top tube lengths so you can compare back to them when considering models you can't sit on. Most other dimensions aren't anywhere near as important to fit as those two.