Msg#: 4516306 posted 12:48 pm on Nov 8, 2012 (gmt 0)
It does vary enormously by site. For my sites it was 336x280 but now 300x250 seems better.
Position always has been above fold. Slightly right or left of center seems to work best.
One problem though in making any analysis of best size / position is that some advertisers only offer ads in a limited number of sizes. If one of those advertisers is paying good money it can temporarily distort the results.