Particle - だけ - only, just
だけ is used to mark a limited amount or extend of something (Not more than). It is only used to mark an upper limit. For example: You can only have 2 cookies, or: it takes just 3 minutes at most.
When used with できる, it is a common way to say 'as .. as possible'. For example: できる だけ 速く 走った (I ran as fast as possible).