Make a global variable, eg. named diameter.
Use ellipse() to draw the circle, using the diameter variable as size parameter, and mouseX/Y as coordinates.
Increment diameter on each draw() call.
Don't forget to call background() at the start of draw().

ahhhh i cant get it to work, can you show me how its meant to look?

See the ellipse() Reference. If that still doesn't help you, then have a look at this question.

Looks like you solved your issue, but for the record, I suggest to read the I display images in sequence but I see only the last one. Why? article. In short, I remind I wrote " Increment diameter on each draw() call." and I didn't mention a for loop...

Yeah, thanks for the help :)