In one of my leagues I drafted a 5 foot 11 RB with great strength and potentially great pass block and have moved him to OL pre training camp. He's 100lbs underweight at the moment.
Will his lack of height/weight have a negative effect on him playing at OL while he, hopefully, develops ?

I once had a really good O-lineman who was 5'10" or 5'11" and he was really talented. However, the problem was that he got injured so often he played in maybe half the games each season. I finally released him after a few seasons.

I'm not sure about how much weight he would be able to gain, I haven't paid attention to that very closely. Someone else could probably speak to that much better. I just know that my guy seemed to get pushed around a lot and really took a beating.

Just imagine a strong guy on the line, but standing half a foot shorter than than anyone else on the line!

I have also noticed higher injury rates when a player can’t match the weight of the intended position. It has been stated elsewhere that there is a 50 pound maximum fluctuation of gain or loss from the natural drafted weight they start at. A 5’10 fullback that starts out 240+ can probably gain enough to be at least a center, but crossing 300 pounds isn’t going to happen. If this guy started lighter than 220 like most rb, he will certainly be a liability. Maybe consider him a goal line fb/te to bulldozer short yardage plays, but an every down starter on the interior line will get overpowered.

I do wonder with the lighter weight tho, will the speed/acceleration bonus that undersized tight ends and defensive ends get against naturally heavier opponents apply to him as well. He could be a good counter to linebacker sized edge rushers at risk of getting bull rushed by the bigger stronger players. That could also depend on your league rules if they restrict positions based on weight to avoid those kind of mismatches or exploit them.