Mobile ad sizes have been added for DFP in the discourse-adplugin.
The most popular (and recommended by Google) ad size 320 x 250 is there, along with some others. Let me know if I missed any. There were also some sizes missing for the non-mobile ad units, so I added those.
@charleswalter @BCHK The update is deployed on your sites now so please try them out and let me know your results. When I switched to 320 x 250 it was rendering blank rectangles, but after an hour they began to be populated. It seems to be normal for DFP/Adsense… /shrug