It's all a matter of personal preference. I guess there are some guidelines you could follow, such as: what do you plan to focus on when playing? If you wanted to get into a solid raid progression guild, you may want to aim for medium-full servers, also if you wanted good PvP, I'd aim for the larger realms, although you may want to look up their battle groups first and do some research on them. But, be aware that you may face some queue times when logging into the larger servers.
If you're just starting out and looking to mainly level, I'd say go to the low-recommended server. All in all, it's mainly a matter of personal preference, play around a bit and see what you're most comfortable with.
I don't really know if that helped lol ^^, but good luck.