I want a testudo graeca and i'm going to buy a little one so is any problem to keep him in a terrarium with 50x50x30cm(19,7"Lx19,7"Wx11,8"H)while he is small?

If he is still a baby, that would be ok. But do realize that Graecas grow a great deal, and will need a big enclosure in the long run. Have you read about this species? It's important to study how to care for tortoises before purchasing one.

yes I have studied a lot about them and I know that they are realy energetic but i'm thinking in let her walk around my room when i'm there, to do some exercise
sorry about my bad english

letting a tortoise freeroam isn't a good idea.
http://www.tortoisetrust.org/articles/roaming.html

I wasn´t thinking in let the tortoise roaming freely , I was thinking to let her roaming beside me

Bascari,
even so, I think Stells is right (check out the link she posted)
best to keep the tort on the terrarium 24/7.
just make sure it has room in there.