As Steve said!!
A huge can of worms!!
I asked about this a few months ago.
IMO If your just measuring TT then do it centre to centre along the top tube NOT the head tube and seat tube ends. It could be a huge difference if your seat tube was at a weird angle and stuck out a lot from the top tube. It could add another inch. Or Dibly's where he doesn't have a protruding seat tube!!
For judging a frame's size ONLY then you have to forget about the rear triangle and Head tube angles and all that other stuff. It important to how the bike performs and feels but hasn't got much to do with whether you fit the bike or not!!
The real difference to frame size and feel is the relationship between your BB and Head tube because you'll be standing up most of the time.
When you pedal the centre of rotation is the BB. If when you pedal your knees keep hitting your bars then you need a head tube that is further away from the BB or in the case of OS stems a Pro, XL ,XXL etc, etc to move the handlebars further away. This really only works to a point because it still leaves your front wheel in the same position. If your 7ft tall and used an XXXXXXXXXXXL stem then most of your body could be past the front wheel which would be bad!!
Heres an example:
I own two Haro's both with a 20.5TT Centre to centre along the TT.
Haro Group One RS1 (late 80's)
Haro Group One (early 90's)
The second one feels longer than the RS1 because the BB is 1.5-2 inches further away from the head tube. Which in turn gives my knees more room and changes my body position so I'm no longer over the top of the bars. It feels longer!!
If you've ever seen an RS1 then you'll know what I mean. The Seat tube is at a crazy angle to make the tt long but because the BB is in the same place then it still feels like riding a 19 or 19.5"!
