9″ is very reasonable and
9″ is very reasonable and will work for most things.
13.5″ will better accommodate large drapes/handkerchiefs and casting. Additionally, glass is less likely to thermal shock in taller kilns as the heat has more opportunity to even out on the way to the glass.
The difference in electrical cost of firing (two otherwise identical kilns) is negligible.
Helios Kiln Glass Studio