I have a love/hate relationship with banner exchanges. I personally used them for a few months but once my sites started picking up a lot of traffic I nixed them. Most exchanges use a sliding scale ration as your page ctr drops, so does the ratio of clicks sent to clicks received. As my sites were new, I burned up through all the free impressions and then sliding scale droped down to 60% meaning I was receiving 6 clicks for every 10 clicks I sent to the banner. This is in a high CTR spot also. So I replaced it and put an ad there to make money instead of loosing visitors. But hey, what works for me maynot work for you and vice versa. A lot of arcade webmasters love them and I know a few guys that run a couple of them. My personal favorite is
http://www.bannerflux.com . The owner is really nice and very fair and gives something like 50k free impressions when you sign up.
As you noticed with the 100x100 ad getting more clicks is because that is the standard size that everyone uses.
Hope that helps!