The max file size for a Google "Image Ad" is 50k. I'd read up on the specs for whichever advertising network you will be distributing on and use their specs as your guidelines. Ideally, your ad should be as small as possible, but 50k should probably be your upper limit. When I first started doing banner ads back in the late 90's, we shot for 10k. But people expect more from a banner ad these days and if you gave most advertisers a 10k ad, they would probably complain that it's not flashy enough. It's possible to find a designer that understands how to design a good looking ad with a small file size, but most feel pressured to produce more complicated ads with animation or too many colors to compress well. You can sometimes get around this problem using Flash (Google does allow flash animated banners) but most people prefer to go with a small GIF for maximum usability with different advertising networks. You can see some examples of some bandwidth-friendly ads I did for our real estate school here if you're interested...there's both Flash and GIF and they conform to Google's "Image Ads" guidelines.