The short answer is "We don't really know but probably pretty lean and agile", if you want a longer answer, /u/Iphikrates wrote a fantastic answere here. As for roman soldiers it's pretty much the same story. While they were, at least for part of roman history, professional soldiers, their training was functoinal rather than optimized for muscle growth and we can probably assume that their physique would not have been wildly different from that described in /u/Iphikrates post.

That said, the average Spartan did not actually train specifically for war all that much. I have never seen a single contemporary mention of forces marches for training purposes, formation fighting practice or unit maneuvers.