1. in the image above, obito seems a bit taller than kakashi. and minato although at the back may not seem to be very tall at all. the height is not really much of an issue at all. when tobi fought minato, he was wearing a cloak (hes real physique is hidden), and they did not stand side by side to chek whos taller or what. if it was obito we could assume that he had the height comparable to an adult. so altough he might still be a teenager that time, his height might just be enough to assume he's tobi.
2. about the sharingan that kakashi possesses, it was already been explained that the reason kakashi always had to cover his eyes is because it drains his chakra extensively coz he doesnt have an uchiha blood. meaning that an uchiha could expose or use his sharingan all the time (as tobi does) coz it doesnt drain much of his chakra. also an uchiha can use the full potential of his sharingan coz its in their blood, that's their main power, unlike kakashi who's not an uchiha (that's why he can only transport a smaller portion or small stuffs through kamui, unlike tobi who can transport anything or anyone, including himself with ease). but basically kakashi and tobi have the same use of sharingan. it was also stated that all sharingan users have unique powers, but in the case of tobi and kakashi, they have the same.
3. when kakashi tried to use kamui to tobi, tobi stopped him, saying don't bother, it won't work on me. could it be that he's saying that, it won't work on me coz that's my sharingan you're using? and gez who gave kakashi that saringan?! obito did..
4. lastly, i had a small theory of my own, that obito might be in all this ever since the beginning. remember he was an uchiha, and he envied kakashi. that insident being crushed by the rocks may just be all part of the plan, him giving kakashi one of hes eyes, pretending to be dying. the body wasnt recoverd at all. being burried under the rocks may just be hes way to get out of the picture so everyone would think he died. and then later on joined his real team, madara and zetsu explaining why he knew everything about uchiha history and what not.
4. among all the uchihas that could be tobi, i think obito would have the greatest impact and twist. i dont know im not sure. but for me he has the biggest possibility among others.