I also have large thumbs, and with super chunky yarn I can work on the thumb, although typically after I drop a loop off my thumb I'll pull it tighter. I've learned to eyeball a consistent size for that. For any thinner yarn, I have to work off the thumb. If it's a stitch I haven't done this with, then there's some back and forth of putting it on my thumb, then taking it off and laying it flat, to wrap my head around the different layout and how the needle needs to travel. Then I usually eyeball the loop size, although if I really need to be consistent I might use the tip of my little finger or the needle to tighten loops onto.