For an ADULT- As big as you can spare the room for! Seriously Hermanns tend to be super active. I wouldn't go smaller than 4' x 2' for an adult, and that's pretty cramped in my opinion.
I have a 6' x 2' enclosure for my adult male (about 5"-5.5") and I often feel like it's too small. My guy gets stir crazy pretty quickly, and I routinely have to let him run around the house to 'stretch his legs'.
These guys would walk for miles and miles in the wild - they're made for walking!
I have my 3.7 inch Hermann's in a 3' x 2' enclosure for the winter. I will have something larger for him for next winter, as this enclosure is just large enough for a hide and cool and warm sides. As he grows, even a little, this really won't cut it any more.
on the subject of enclosure size can I add to this thread with out "highjacking" what the thoughts are on substrate for the enclosure? what is the recommended depth? my wife wants to pick up an adult female about 8" she is working on building a tortoise table not sure what dimensions she is working with but I know the general rule of thumb is that the walls should be a bit larger then the tortoise on its back legs plus substrate depth. it is her project i'm suppose to stay away but i'm trying to assist with out taking over her build.
As Pokeymeg stated, you can't hardly make an enclosure, inside or outside, too large. My inside enclosure as about 6' X 3' and, for 4 "pretty much" adults, that's barely adequate, and on sunny days, they go out on my porch for some sun and to stretch their legs a bit.
Building them an outside enclosure of about 16' X 8', to replace their current outside home of about 8' X 5', which will then become the nursery, once eggs start hatching.