While mine is dynamically allocated, I have it set to 64GB. I find it a reasonable size to fit Windows 7 64-bit plus updates and software I need to run and files that get moved around (typically disk images and such).
For RAM, usually 2-4GB, I've got 8GB RAM and an SSD, so it's no issue.
I store the VM on my SSD (256GB rMBP) but I also have it on my external USB 3.0 4TB drive and it's almost as quick there. I had it on my internal hard drive on my 2008 MacBook because I had a 120GB SSD and a 750GB HDD, but that was when you could have two drives 😎
Smallest I'd ever go is 32GB, but you may run out of space trying to install updates or software. I have 14 PCs I built for check in kiosks at a church running 64GB SSDs, 4GB RAM, dual core Pentiums, and they are around half full or slightly over it with just Windows 7, updates, and a small kiosk program.