It will become much more obvious to you once you start doing it. There really aren't a lot of US classic stamp sizes, but a few are best done by trimming strips instead of buying pre-cut. 95% are probably 22x25 or 25x22. I have a Showgard US Combination set of pre-cuts (which has way more jumbos than I need) and replenish the 22x25, 25x22, 25x40 & 40x25 as I use them up. There are a few 27 high early issues and fewer still 23 high (but wide) I cut from strips. If you're going to do things like Souvenir sheets you might as well just measure them, it'll take the same time as looking them up. I recommend the Precision US Specialty Multi-Gauge for measuring, which you'll want if you're doing US classics anyway.
Your individual stamps may also vary in size from those in the reference material if you have jumbo margins or even one or two margins larger than average or if you have selvedge attached to the stamps. I generally used 25, 27, 28, 31mm for most of my US classics.