Ad position is more important than banner size, but assuming you're talking about banners above the fold (ads that the visitor will see without scrolling down) both a 768x80 leaderboard and a 300x250 or 336x280 rectangle will have higher click through rates. The 160x600 skscrapers on the right side of pages are ok, but the leaderboard and rectangle are superior.
As for results and testing, doing 1000 impressions on a bunch of sites will provide no meaningful data. There is a ton of randomness in online advertising and you really need the highest amount of data for the lowest number of options. This is also a tough question to answer because we don't know your market or your goal. Are you trying to gather leads? To make sales? You're also testing two things: the click through rate (CTR) for your ads and the conversion rate of your landing page or website. For both tests, you need lots of data, but the second test is somewhat dependent on the first.
Maybe try this: Get 100 visitors to your landing page. My guess is that if your ads are well constructed and the sites are well targeted you will need to run 3k-6k impressions. You will hopefully have a CTR of 2-3%.