Remember, a short board will be strangely light and loose in comparison to your longboard, so you don't want to go too short at first. Don't go too small or thin. You will have a hard time catching waves and will bog down in weak surf.
In terms of length, from 6'10" to 7'2" is a good range depending on your size. Regardless, be sure your board is at least one head taller than you. Around 19-21" wide is solid but be sure the surfboard has plenty of thickness to float you (around 2+ inches thick is average).
If you want sharper shortboard-style turns then avoid funboards (wide surfboards with rounded long board style noses). Try to stay within 19-20 inches max. However, funboards will give you more wave catching ease, so think about what's most important to you. Many longboarders ride funboards, but they aren't technically considered "short boards."
You local surf shop might tell you differently, but a fish surfboard is not the best choice for a transition. Sure, fish surfboards are fun, but these surfboards' wide tails and quirky fin configurations can be loose and squirrely and really require a higher skill level and take some getting used to.
Shucking out cash on a new board is only fun if you love it, and if you make sure your surfboard floats you well enough to catch waves and is designed to surf the way you want, you will have an easy move from long board to short board for sure.
Any of this surfboard terminology confusing? Learn about the parts of a surfboard.