The maximum avatar size is 140x140 and must be < 70 kB. TrekBBS now hosts your av for you and can upload it straight from your computer.
To do that, go to the top of this page click on the "User CP" (User Control Panel) link, then select "Edit Avatar" from the menu on the left. That will take you to a page where you can upload your av.
It's fairly straight forward, but if you need any more help just ask.