I like an 18 inch round hoop for hand quilting. There's a balance to be found between too big and bulky for easy management and too small to get going on a motif or line before having to reposition the fabric. For me, the 18 inch is it.
One thing to remember in learning hand quilting is that you don't have to go for the smallest stitches ever right at the first-- it's better to get consistently sized and spaced stitches. Over time and with experience, you can work toward the ideal tiny stitches.
Another tip I find essential -- wax your threads. Saves so much frustration in tangled and fraying thread.