The magic lantern team is known to be tireless in making canon cameras rocks. I am sure you all are doing your best for raw.
So here is a list of widths wich are close to the maximum resolution 600D can handle without skip frames or magenta frames in 16:9
856, 854, 852, 850, 848, 846, 844, 842, 840, 838, 836, 834, 832, 830, 828, 826, 824, 822, 820, 816
a tip would be to start from the highest values and go decreasing until find which one works good.
many thanks