it matters what size you want it.

Take forumotions banner for instance
that is 835x156 i mean you dont want to make it to wide or it can make the forum off set or some members would have to to user bottom scroll to move the page over.

And it can depend on people monitors too.

For width to be safe i would not go over 1024.. and thats kind of long.. Between 800 and 900 is usually the best, but it depends on your forums page dimensions as well.
So whats you banner and link i can resizing for you and or help out for what you think would be best for your forum

I don't have a banner yet. I was going to make a request in the graphics section but I was not sure how many pixels to put down for the dimensions. Do you think it woudl be okay to link you to my site and get your oppinion on how big I should request for a banner?

If you'r okay with doing that, here's the link.

Spoiler:

http://adventuretimerpg.forumotion.com/

Thank you for your help by the way.

since your using widgets on the side i would go with the same size as the Forumotion banner may be knock height up to like 160.
i think a little thicker of a banner would go good at your place.

pm me the link to your request after you post it, I'll take a look

Okay, thank you verymuch for all of your help. I guess this is problem solved.