Each gecko is genetically different, so their "adult" size varies a great deal. It is far easier to go by age. If you buy a juvenile/baby gecko, and do not know the age, then consider your first day of ownership to be their first day of life.

So I would guess that yours are probably full grown. Of course, they may get wider though! LOL
I would suggest making the hides and other furnishing under the assumption that they will be roughly 10-11" full grown. For most "normal" leos that would be pretty darn big, but better to have a more roomy cage than a cramped one! You just never know how big they're gonna be
Julie