720p should be no problem, 1080p might stress the CPU too much but could work.
Anyway, 720p will look better at lower bitrates, and a lower bitrate is better for more possible viewers.
General settings:
Bitrate: 2000-2500
Base Resolution: 1920x1080
Resolution Downscale: 1280x720 (I think thats 1.50)
FPS: 30
That should be a good starting point for LoL. And good thing is, OBS is free, so you can just test it :)