the title says it all
i want it to be banners not bigger that 300 px in height and 600 px in width

I am not sure what banners should refer to.
Do you mean images in signatures?
If so, there is no way to set an automatic limitation.
You should post the rules somewhere on your forum and watch if they are follwed. If not, you can ask the user to change it, or change it yourself via the Admin Panel.

I'm sorry, I know what a banner is I'm just not sure where you want to put it.

If it's in your signature, you can't regulate, but you just have to make a "rule" with your requirements.

If you want to resize a banner, you have to do it in a program like Photoshop or Jasc Animation.