Here's a good link. You can use the smallest, then put them in the larger ones, then into gallons and so on, without having to remove anything, they'll just grow right though the bags.
Or start with the size you want. The smallest ones are best when your trying to put together a tray of plants, and want them under one small veg light, cause they don't take up much room, and they have really good air flow. I've still got plenty of them up in the closet, but I've been using solo cups lately. lol Both work well.
I'd say the bags are a bit better at getting stuff started, and might be better suited for doing cuttings, but dry out faster. Whereas the solo cups, and shot glass cups are better at building up a really tight root mass. The bags make a really highly (powder puff) root ball, that's going mostly outward, and the cups make ropes, where roots go round and round the outside, with lots of runners. Two different techniques, no doubt. But both work very well.