I usually try and space the smaller ones about a nickel or penny's breadth apart, just so they can get to full size with no trouble. I'll plant large pots this way at my mother crop, because I find it easier to collect gemmae from plants that aren't packed too tightly. The rows look kind of unattractive, though - I'll usually save some extras to scatter randomly in smaller pots, so I can have some of those charming little clumps of plants. If it's your first time growing them, though, I think it might be advisable to spread them thinly so that your first generation has minimal competition. Once you're drowning in gemmae it'll be easier to do whatever is aesthetically pleasing.