it's something about resolutions? or what? any examples?

"This additional theme is open to interpretation."

So "Pocket Sized" means whatever you choose to interpret it as.

Maybe the game is pocket sized? Maybe the game is about something pocket-sized? Maybe the environment is pocket-sized?

At least that's my interpretation :)

It's intentionally vague so as to be open to interpretation. Is it small resolution? Sure, if you want. Does it mean a mobile game? If you think so.  

Think about the words and decide what you want to do with it.

Here is an idea: each tile/character is one pixel wide and one pixel tall. ;)