Originally Posted by Hiro11

Knowing your preferred stack is a good starting point here. I largely ignore t-shirt sizing and seat tube or top tube sizing and just focus on stack and reach. To determine the stack and reach, find a bike that fits you well and either look up or measure the stack and reach on that bike.

Even knowing your stack and reach don't solve all problems. For example, my Giant Defy has a very tall measured stack. However, Giant has taken almost 15mm out of the fork length on the bike when compared to similar endurance models like the Domane. This makes the Defy actually have a more aggressive front end than a Domane or Roubaix despite what the stack measurement might lead you to believe. Similarly, a reach measurement doesn't account for the effect of the stem length and bar reach of a given complete bike. Stem length and bar reach can vary widely. For that reason, I also measured and memorized my preferred "true reach" from the nose of the saddle to the center of the lever hoods which allows me to determine if the stem and bar on a bike I'm considering are going to work for me. Snub nosed saddles have now screwed up even that measurement. Gah.

One note: Canyon's sizing is unorthodox, their bikes are generally significantly larger than the same t-shirt size on another brand. This is annoying given that Canyon is a direct sales brand and you can't try before you buy. BMC also has wacky sizing.