The most common way to hang them is with a sleeve. For a wallhanging I would make one no wider than 2 inches and use a small rod. I sell many quilts and many of my customers don't want to install the hardware associated with the rods. In those cases I either sew a loop on the back so they can hang it with nails or hooks. Lately for many of my sold quilts I make holes with those rounds thingis (sorry, don't know the name, but is the thing you use to make the holes for a drawstring like on a hoody) and they hang them on the wall with decorative nails. I know, That damages the quilt on a permanent way, but once is hanging you don't see it. I usually show all three possibilities and my customer chooses.
As for the size, there is no rule of thumb. I usually go by the size of the wall where it will hang and what else is on the wall. You don't want a small quilt on a huge wall or the other way around.
Maria