Flatlander, if you're using the foam dink floats buy the kind with the high density foam instead of the low density ones. The high density foam has smooth sides but the low density foam looks like it has bubbles in it. The high density feels harder than the low density when you squeeze them. The high density foam lasts a lot longer when you wrap your line around it, as the line doesn't cut into it when you snag up or fight a large fish. It also seems to slide less up & down your line than the low density foam.
When buying your floats get the kind that have the plastic straws at each end pointing at an angle out of the sides (usually about 1" from each end) instead of the straw going through the middle. When putting the float on your mainline, put it through the top straw, then once around your float, then through the bottom straw. Once you have the float positioned where you want it (and after adjusting for water depth), give the mainline above and below the float a bit of a tug; this helps the float from sliding up and down by snugging the mainline against the body of the float.
With high density foam and the tug, I find I don't usually need any stopper. But just in case, I keep some toothpicks in my vest. Sometimes a well used float starts to slide. You just need a toothpick in the top straw. Break off the toothpick about 1/4" above the straw so that you can pull it out when you have to move your float.
Size of float depends on water conditions & depth, how much weight you're using, and the fish you're targeting. The size I usually use is the 3/4"x5.5". I never use the 1" diameter floats. In slower or clearer water I'll use the 5/8"x4" size. When fishing for pinks in the upper Fraser I use a size of about 1/4"x4" because my weight's only 1/4 ounce and the fish are in shallow water.
I hope this helps you.