1. Yes 500GB should be more than enough, but you should also consider that a TB doesn't cost a whole lot more.
2. Windows 7 can grow between 15-20GB, I don't think Arma takes up more than 10GB. You should be ok with a 64GB ssd, but go bigger if you can.
Thank you. I know it would make things simpler, but would it also be effective to just get a cacheing ssd (maybe 50 or 60 GB) ?