Have a 3 in 629 and now kinda want a 629 2.5 inch. Thinking about the 2.5 for ccw. Is the size of 2.5 inch basically the same size as the 3 in 629 except for barrel lenth?

Yep, I believe it is.

Maybe someone else can confirm, but I think most 2.5" are round butt frames & most 3" are square butt. Square butt grips are larger & less concealable than small, which would be another size difference.

Every factory 3" 629 I've seen are round butt, actually have never seen a square with a barrel less than 4". Of course I haven't seen all the 3" guns out there so it just might be my limited exposure.

Both the 2.5" and 3" have a round butt, but come with a round-to-square Hogue rubber monogrip. Both have shortened extractors, but perhaps not the same length.

Round butt grips not only conceal better, but distribute the recoil better in your hand. I put boot grips on my 3".

I doubt the 2.5" is any more concealable. It's the cylinder thickness and butt that affects concealability more than barrel length, once you are at 4" or less. If you crave another short 629, who am I to persuade you otherwise. I'm happy with 3".