Fedor isn't scared of anyone in the UFC. To be fair though he has never faced a grappler that is bigger than he is and he would if he was fighting in the UFC.
Fighters that Fedor has faced that had a significant size advantage:
Hong Man Choi
None of these has a ground game.
Good grapplers that Fedor has fought:
Big Nog (Fedor's best win IMO. Nog was the best HW in the world before he fought Fedor)
All of these guys have been about the same size or smaller. Arona, Randleman and Babalu are really LHW's and Lindland is a MW. Mark Coleman is now fighting at LHW. So that leaves Nog who is around the same size as Fedor and Fedor couldn't finish him. Just sayin.
There are a couple HW's in the UFC that could give Fedor problems as he really hasn't had to deal with someone that is bigger than him AND can grapple.
I know guys like to really emphasize size advantages, but honestly, at this level it doesn't mean a hell of a lot. Even at the amateur level it isn't that big of a deal, at least not in my experiences.
You can go ahead and poke holes in every fighter that Fedor has faced. HMC didn't have a ground game. Fedor fought Nog and only dominated the entire fight without "finishing" him. Randleman - who is easily as strong as a heavyweight - is really small. Whatever.
It's easy to poke holes in fights. I could mention that the UFC's heavyweight division is thinner than the Diaz brothers and the wins that Couture has gotten are nothing but a result of that - the man couldn't even finish Tito Ortiz, after all
- but what does that prove?
The fact is, Fedor has won A LOT of fights and he has proven himself many, many times over. The range and depth of the types of fighters he has fought and beaten is unheard of. You can say that UFC fighters could give him issues because some of them are bigger if you like, I don't think that, but since it's not really knowable, maybe it's a better idea to observe what has happened as opposed to what one can imagine happening.
Nog is not "about the same size" as Fedor. Big Nog is 6'3" and Fedor is barely 6'0".