Ponds come in all different shapes and sizes. The shape and size you choose completely depends on your personal circumstances (location/landscape/budget/etc). However it's really up to you how big or how small you want your pond.
Of course, the smaller your pond the more limited options you have in choosing your plants and fish.
The size of your pond will also depend on any additions you had in mind as well. For example, adding a waterfall or a fountain. Your pond will have to be large enough to incoporate these additions in such a way that there will be little to no water lose. This means that you don't want water to be splashing out of your pond otherwise you'll eventually deplete your pond of water.
If you're not looking to add any water devices, fish, or large fancy aquatic plants then your pond can be almost any size you want.
