Bigger IS better. If you can afford it, get the biggest (or biggest you can afford). Whether you use it immediately, or not at all, the extra money will be a passing regret. Living with a device with too little storage will haunt you until you can afford to correct the error.
If there is any chance you'll take it out of the house, get the cellular version. You don't have to activate an account, but you'll be able to if you ever need it, and right on the iPad. Perhaps the bigger issue is the GPS which you won't have if you get the Wi-Fi only version. It is surprising how much easier it is to use a larger screen map, and the GPS enhances the usefulness of many other apps.
Find my iPad is far less useful without the GPS.
But most of all, stick around and read through the form for a while to see how others are using their iPads. Base you decision on what interests you, not what you are sure you'll do. Leave yourself room to grow.