App working or not...The common sizes appear to be 4.5", 7", and 9". I know there are others. I have seen 4" & 5" for example, but sticking with the common sizes seems to make sense. I use 4.5" a bunch, but when I need real power I grab the one 7" Makita I have. That is a beast. I don't think I am big enough to handle a 9" grinder... All of my 4.5" are Makita except the one Northern Brand I got. That was the 2nd grinder I got after my first Makita (It was very cheap). And that is why all the rest of my grinders are not Northern brand, but are Makitas... Totally gutless, but then you look at the amp rating, and you know why. I use it only for lighter tasks. I think I have 4 grinders at 4.5" and the one 7". It is close to enough grinders for me, but not quite...