I definitely agree with you, discounting guys because of their size is a big mistake in a lot of cases. Guys can bulk up to 230 or 240 and be effective, that's not an issue in my mind. It can be a problem when you're 220 and have to constantly battle 300 lb linemen and hit 220+ RBs. There's a reason many true ILBs or MLBs are in that 240/250 range, that 20 lbs of muscle can help you withstand the pounding.
In the case of Lee and Cravens, although they are very different players, I don't see them posses those great ILB instincts. A better position for Cravens would be an "in the box" Safety or as a SLB in a 43. Lee belongs in the 43 as an OLB, again in the same vein as Julian Peterson. I think he could play ILB in a 34, but I don't think we would see him fare much better than Brad Jones (i.e. Very miscast and not as effective as he could be). I don't think he's the same as Shazier or Thompson, those guys are very natural LBs.