I believe the recommended size for a grinder, is 13x13. If you're just starting out however, I'd suggest making a cacti farm first. While they're a pain in the backside to make, they're a lot cheaper and will get you roughly the same kind of income, if not just slightly under it.
This is just recommended for starting out, if you feel it's easier to go straight onto a pig farm then you're more than welcome! Just another suggestion, as I know it's quite a grind to get spawners, especially when you're new-ish to the server.