I prefer a bit longer sanding block. I use ne that is about 5" wide and 10" long. You can wrap one full sheet of sandpaper around it and hold it in place with your hand.

The benefit of having the longer sanding block is that it keeps you from sanding humps and bumps into your edges. A short sanding block can sometimes lead to a washboard look in your edges.

For sanding balsa. Just start out with 80 grit and finish with 120 grit.

You could make a sanding block from 3/4" particle board; available at any home improvement store. Cut to desired size with a table saw or jig saw.

If you want a consistant taper on your edges; just use a shim to prop up your trailing edges stock. For instance; use a 3/4" thick piece of wood under the side you wish to sand-- then sand with your block-- always keeping your block level to the table underneath.

Hope this helps.