The most popular and highest volume ad sizes are: 320x50, 300x250, 320x480. For MoPub campaigns Native Ads should also be included (they show up in Inventory as 0x0 size).
Intermediate volume ad sizes are: 216x36, 300x50, 480x320, 728x90, 768x1024, 1024x768, 480x80 (Windows Phones), and 640x100 (Windows Phones).
You can also upload creatives in a variety of other sizes. We always recommend that you use the Inventory Tool to see how much traffic is available for your chosen creative size.
We also recommend sticking to one creative size per campaign so you can optimize based on performance of each creative. Banner size can have a major impact on performance, and we recommend testing as many banner dimensions as possible.