I'm primarily a third baseman, but I could be playing some right field next season (the opposite of what you're doing just kinda funny). It depends on what your coach wants you to do. At my school we almost always play even with the bag, just in case of the bunt. As for the glove, a 12 isn't uncommon. I use a 11.75. Usually middle infielders like 11-11.5 and third basemen like 11.5-12.
A big part of third is knowing what's going on. If you have an un athletic lefty on the mound and they are going to bunt, then you probably should play a little closer to get a better jump on it. Also, a big thing for me, know the range of your shortstop. Mine is seriously athletic and has crazy range, which can allow me to play closer to the line. If it's an in between ball, I've always been taught get to everything you can get to. On the throws make sure you set your feet, sometimes I rush myself and throw off balance and it can lead to bad throws. Sorry for the long response! Good luck man!