I dont think you have to be bigger to play DE in a 3-4, what im saying is that it would be a waste of his talents to do it, you dont ask your DE's to get after the qb, its more about taking on blockers which as I said would be wasting alot of Dorsey's abilties.
As far as Jay Ratliff, yes he is smaller, but for 1 we moved him back outside to DE, but even when he was playing NT along with Tank, both guys were out of position and not big enough which leads me to my original point, yes certain guys "Can" play NT, but they are not "true" nose tackles, sometimes you work with what you have...Dallas has not had a true NT since we started the 3-4 and it shows, now by "true NT" I mean the prototype, not in terms of height but ideally you want guys over 320 but of course guys under that weight can play I never said it cant happen unless your a certain size.
You mentioned Franklin for SF, he is another case where he is playing there because he has to, if you check with SF draft boards they are in the market for a "true" NT also...Ferguson is not a true NT either, but he is solid and better then anything you guys had so he is a solid guy to play that spot, still does not change the fact that if you specifically draft a guy to play NT in the 3-4 he's going to be alot closer to the prototype, look around the league, any team who has an undersized 3-4 NT probably struggles in their defense(Cleveland, San Fran, Jets) and the ones who true NT's(Pats, Chargers, Steelers) all perform better, teams will plug in who they have to(as will Dallas this season) but we still dont have the ideal guy in the middle.
As far as Dorsey playing DE, its not about size, its about his responsibility in that role and the fit for the defense...Dre Moore makes alot better 3-4 DE, Red Bryant makes alot better NT, but Dorsey is the far superior UT and probably the best one to come along in the past 10 years, but just because he's an elite talent you cant play him out of position otherwise you will not reap the benefits you expect from a top pick, ie Oakland playing Michael Huff at SS.