Agreed, his size and the resulting hit to his defensive potential is why he hasn't been receiving high major offers yet. Which is why IMO it's worth taking a flyer on him and letting him redshirt for a year. Maybe he grows a few inches, maybe not. Maybe he adds some muscle, maybe not.
The truth of the matter is, Iowa actually appears to have some PG depth the next few years, especially if JBo misses next season. That would mean potentially having 3 PGs on the team in 2020-2021 that Fran can rely on while he lets BB redshirt. If BB develops great, if not, really no harm done. Fran will know what he has in BB and still have time to go out and find Connor's and Joe'Ts replacement if BB isn't up to the task.