I think that ultimately, muscular strength has little or nothing to do with one's ability.
Look at O'Sensei in his later years, or any of the older Shihan now. They certainly weren't/aren't buffed-out bodybuilders but they can do some amazing Aikido.
Internal arts, from what I can tell, are all about using a minimum amount of force to effect your goals. Cheng Man Ching used to talk about using four ounces of strength to move a thousand pounds of oncoming force. I mean, the most basic of Aiki techniques is just to get out of the way and let uke continue where s/he is was going in the first place. That takes nearly no strength at all...just a step and a turn.
As for size and weight lifting... I don't know. Height can give someone an advantage on some throws (iriminage), but being shorter than uke can help out in some cases too (shihonage). If you're doing weights for muscular size I think it would be a hindrance to the softness, flexibility, and inter-connectiveness of your tissues. But if you lift for endurance and flexibility, who knows, it might be helpful.