size, but like you said, his lateral quickness is suspect, & his length is pretty poor. Of course his length would be a concern in the paint as well, but so long as we factor in overall length from from Gavin & Haarms & the space-making from Harward, I'm not so concerned about failing down there.
In the end, I think Lowell's athleticism gives him a better shot holding his own at the 4 than the 3.
But that's just a pretty wild hunch on my part. I've seen only a little UVU & HS film on Lowell.