Win 7 size within VMWare
I'd go with VMWare fusion's defaults for Windows 7:


60 GB drive
1 GB RAM


Unless the memory on the guest OS becomes very constrained, you might only need to bump it to 2 GB to get a little speed up if needed. The 2012 iMac CPU is quite capable and you do have significant amounts of RAM so you could juice the VM to a lot more RAM if needed without starving OS X for memory.