Daniel;
Being picky; both settings are not equally divisable by 8 (or 4) on both axis.
I guess it was a typo on your part but the horizontal dimension on the first setting (854) is actually larger than the source dimension (720). Enlarging the video is last thing you’d want to do!!
Anyway, back to Jeffreys post –
Export out of premiere at full resolution – 720×486 using the source settings.
For the 300Kb/s stream I’d look at a frame size of 384×216, or if you want to push it 448×252 which is divisable by 4 but not 8.
For the high quality download I’d plump for a frame size of 512×288 or 640×360.