Float size isn't as much of a big deal as keeping the float neutrally buoyant. Like Kechin said, Crappie can be finicky eaters and hate resistance when they aren't hungry. You can entice them to take anything into their mouth, getting them to hang onto long enough for you to set the hook is another story. The closer you can keep the bobber to neutrally buoyant the better. I prefer to add weight (split shot) about 12"-18" from the hook. This serves two purposes.
1) Add enough weight so that a lively minnow can move that bobber up and down and all around.
2) The minnow has some tether length that they can swim up or down or all around and really entice the crappie to bite hard. The weight holds them to a depth, plus or minus the distance between the weights and the hooks. If your fishing 8' down with a 12" tether between hook and split shot, the minnow could be anywhere from 7'-9' down. Around here, crappie love this depth.