I think you need to get out and try some different bikes to get a feel for what suits.
A lot of your decision will be driven by the type of riding you want to do.
As Danny says, if you want something intermediate go 24", that way if you go street style you can get an Inspired or similar to do the spins and bunny hops on.
If you want to try TGS then 20" / 26" are more suitable but will take more time to practise and learn on. Don't be put off by 26" being big and long, if you want to ride a bike like that and put in the time you will learn.
I started on a mod and it was great for basics, but as my riding progressed I felt it too small and limited. I moved to stock, got riding with a bunch of lads who helped me with technique and it felt much better and my gaps etc became a lot bigger.