These 8 inch/20cm spools are the next best thing to proper deep-sky reels. They're light, easy to carry and inexpensive. They hold plenty of line, and wind in a lot per turn. Letting line out with drag is easy, and the grip is comfortable for any length of time.
I cannot give a capacity with any certainty, but for me 8 inches is the most ergonomic size for winding in. While 10 inches is a little too big, but still manageable, anything smaller than 8 inches is definitely too small for me.
Used properly, these spools can last for many years.
Snap swivels are strongly recommended
Note of warning!
Winding stretched line directly onto any spool, reel, or winder - that is, line under tension - can destroy it.
Instead, take up the pull with the flying hand so the tension is not passed onto the spool. Allow just enough tension to keep the line tidy. Otherwise, a potentially destructive amount of compressive force can build up, as seen on the plywood winder at right.
Light, thin lines can be more destructive than heavier lines. Being thinner, spools hold a lot more of it. Stretchy lines are not only worse for their destructive pressure on reels and spools, they also make kites much harder to control.
Kites are controlled by pulls and releases of the line. Rather than transmitting line tugs and releases to the kite, stretchy line merely stretches or contracts, and control inputs are lost before they ever reach the kite. Loss of control can lead to a lost kite.