This is a personal decision and is totally dependant on how much you intend to compost. Make sure it is not too high for you to fill comfortably. There is no point having a container you cannot use properly. Keeping a lid on it Keeping the rain out is essential. Your composter should have a lid that can be removed and replaced.
Compost bins are usually open at the bottom to allow liquid produced in composting to drain away.
Gaps in the sides It used to be said compost containers needed holes in the sides to allow air flow into the compost. This in fact just dries the compost out at the sides. Enough air is usually mixed into the heap when new material is added, and as it is turned through. The top should open up wide enough to add and remove material, and to allow the compost to be turned comfortably. Alternatively, having a removable front is ideal.