Columbian's are about 25% larger than Richardson's.
When ground squirrels emerge in the spring or go down to estivate and then hibernate for the rest of the year depends mostly on latitude & elevation, but also on sex and age. So it can vary considerable even within a given species throughout its range.
Diggers and predators have been a main hunting interest for over 5 decades for me, so I've had the opportunity to hunt dozens of species over the years. My main stays with diggers the last couple of decades that I've lived on the Mojave Desert has been Belding, Whitetailed Antelope & Rock GS's, prairie dogs and rock chucks. FWIW, the Belding's spend the most time underground of all our NA GS species, so their season can be quite short. But they usually occur in large colonies with such high numbers that shooting can be fast and furious from sun up to sun down in a good area, and as far away as you can see them. WTA's are among the smallest in NA, seldom occur in what could be called colonies or in any kind of numbers any place I've ever found them, and often have to be shot at long ranges. So they can be one of the most challenging. It's not unusual to spend an entire day covering many, many miles to see only a few of them, and a good day hunting them may only be a half dozen to dozen shot.