Back in 2008 we tested 3 banner sizes on one client and the ugliest one won by a mile
I was at a small shop in Columbus doing display for a regional bank, and we ran three banner sizes across the same website for six weeks on a $4,000 budget. The big clunky 728x90 thing nobody on the team liked pulled 2.1% click through, while the slick 300x250 we all fought for got 0.6%. I kept the spreadsheet pinned to my cube wall for a year because I could not believe the numbers. My creative director just shrugged and said "people read what they can't miss" and went back to his coffee. These days everything is feed and stories units, so those old banner lessons feel like ancient history to the juniors on my team. Anyone here still run straight display banners, and did size ever surprise you like that?
That 300x250 probably didn't lose because it was small, it lost because of placement. Banners above the fold in your main content column almost always win, no matter the size. The 728x90 just happened to sit where the eye already goes.