Although it could be a feeble attempt, I heard many fellow photographers that send different sizes to different agencies. The idea is to limit cannibalizing your sales from agencies that pay well.
The drawback would be that you could lose some XL-XXXL sales (from lower paying agencies), but that would include also sub sales for this size. For example Fotolia - how do you feel getting $0.26 for a US sale at XXL size?
How do you balance it?

No, because it's a lot of work already to submit to 20+ sites and keep track of submissions. No need for more hassle, for a yet to verify possible advantage.

x2 ( less agencies - 13 )