I have gone with the 336 by 280 as I find this to work well on both desktop and mobile (mobile users make up about half of all users on the site) in terms of size and exposure. The settings are text ads only on synchronous setting whilst allowing third party advertisers with user-based ads disabled. I have image ads disabled because I get the impression these days lot of cheap image ads are getting through.
Google certified networks on as are general and sensitive categories.
Today's ad fill rate setting is at 74% with 100% earnings.
Disregarding ad size, would you say all other settings were good.