dabac said:
Easiest for wheels is simply to read the tire size that's almost always stamped into the tire sidewalls.
The most commonly used measurements for the frame is from the center of tye bottom bracket - the point thst the cranks turn round - to the top of the seat tube. And the center-to-center distance from the seat tube to the head tube - the part of the frame that the fork turns in.
Right about the tire size. The ISO size is the diameter of the bead seat of the rim, in millimeters. For road bikes, 622 (700C, also know as 28" in Canada and parts of the UK, also known as 29" in the mountain bike world) is most common. 630 is the ISO size for bike-boom era 10-speeds with 27" wheels. The relationship between nominal size and actual is often arbitrary.
Tire width is the second number in the nominal sized. Say, if your tire says "700 x 23C," that implies a 622 mm bead seat diameter and a 23 mm tire width. Nominal width numbers are not necessarily accurate, so the most accurate measure of this is with a caliper.
Dabac's frame size method works only if this is a traditional frame with a top tube that is un-tapered and dead horizontal. But even in the old days, some bikes were measured to the top of the seat tube and some (mostly Italian) to the center of the seat tube. The C-T measurement was generally about 1.5 cm larger than the C-C measurement.
These days nominal frame sizes of bikes with sloping top tubes are virtualized. That is, if you buy a 58 cm Specialized Tarmac, it should fit like a traditional bike with a 58 cm seat tube. Note that there's a lot of variability in nominal size here. For example, a 58 cm Tarmac is two centimeters taller in the head tube and almost 0.8 cm longer in the top tube than a 58 cm Trek Madone. Note here that we're talking about virtual top tube length (horizontal distance between seat and head tubes), not actual. You can usually arrive at the bike's nominal size by drawing a horizontal from the center of the head tube to the seat post, and measuring the distance to that intersect from the center of the bottom bracket. But this doesn't work for many newer bikes that have extended head tubes, purportedly to make the bike more comfortable.
That said, there are two measurements, stack and reach, that are the most relevant to knowing how a bike will fit. Stack is the vertical distance from the center of the bottom bracket to the center of the head tube, and reach is the horizontal distance between those same points. That said, most bike brands don't include this data in their catalogs. But, knowing that variance in head and seat angles of most similarly sized and purposed bikes is pretty negligible, you can ascertain fit from top tube and head tube length.
Any questions?
