Every squad "can" have a banner and the site would handle it just fine. The problem is getting people to design the banners or unit badges.
If you guys want unit badges they should be the same size so that everything is uniform.
For actual banners the size needs to be 728x90 ..
Each squad can have their own discussion area as well in the form of groups, I will get you the dimensions for that.
Here is an example of my squad's banner:
These are the banners that rotate every 5 seconds in the top part of our forums, they are clickable and take you right to the unit's forum.