Have u been reading the same manga as everyone? if so then u would know why he was bigger when he fought the 4th hokage... he was in the shell of the spiral zetsu plus the cells of Hashi. Being in that shell made him look bigger, its abit like putting ur phone in a phone case, it appears to be bigger than it is. So that is the case with Obito when he was in the body of the android spiral one. get it? if not ask Kishi, I will give u his phn number soon, that is if he even picks up, he might drop the phone on ur head loool
However, going back to Kakashi's eye; I dont think he is going blind because he hasn't been rash on his eye like Sasuke or Itachi... he rarely uses his MS, most times he is just on normal 3 tome sharingan apart from when he tried to kill Diedara. like tobi said, if over use the sharingan meaning MS it starts to loose its sight... even Itachi explained that the more u use the huge powers of MS the quicker u will loose ur light which is the cost of obtaining such eye prowess. We can all agree that Sasuke was over using his eyes in a very short period... by fighting the Kages as soon as he got it, and then Danzo and then against team 7. If he rested it could have recovered a little bit but he just decided that there was no point resting since he had Itachi's eyes at his disposal.
If u consider people that had MS in the story, it took years and years for their eyes to actually loose its light apart from sasuke who was too rash on it.