Where did you get the 48x48 limit from? Unless it was changed recently, the limit is larger than that (64x64 I think).

If you mean the file size - make sure it's in indexed mode rather than RGB, makes for much smaller files.

How do I make the profile pic bigger like yours? mine is 48x48 wich is the maximum size, but is still smaller

Didn't know about that either

You should be able to just edit this one without submitting a new one.

Thanks for the advice, I already submited the changes

On PJ, you're encouraged to use a transparent background rather than a solid filler colour, unless the solid background somehow improves the piece (see rule 4 on the submission page). I personally think this would be better with a transparent background. If you really like the green, maybe you should expand it into a background that's more than just a solid colour.