They do vary. My Julies are smaller than my peppered by about .25-.5 inch. In a 10 gallon over several fish that does make a difference. Pygmy's are honestly the best choice for a 10 gallon tank. The big thing here is to keep in mind we are talking about a 10 gallon tank. In a 30+ tank the length won't matter too much. In a 10 it will if you want a group, which with cories you do.
Some common sizes:
Pygmy "habrosus" 1.3"
Panda: 2.0"
Julii: 2.2"
Trilineatus: 2.2" (both julii and trilineatus are often mixed up with each other)
Sterbai: 2.6"
Peppered (paleatus): 2.8"
Brochis Spendens (Emerald): 3.1"
So yes they are similar in size, but not the same. In a bigger tank... who cares