There's no right size... try get the longest bulb that will fit on top of your cage. My male's cage is larger, so I use a 36" for that one, and the female's smaller cage has a 24" on top.
As for the UV rating (10.0 vs 5.0), 5.0 is usually adequate for a Veiled, but on some of those dark, thick mesh Flexarium type cages - which cut out a lot of the UV light passed through into the cage - the 10.0 bulb is sometimes recommended.