The most obvious way to choose the right size pump for your fountain is to look at the size basin it will be sitting in. If you're creating a small table top fountain, you will choose a very small fountain pump that can be tucked away underneath some decorative river stones. Consequently, if you are installing a large fountain feature in your backyard requiring many gallons of water to be pumped per hour to circulate it through the fountain, this will take a much larger sized pump. But there are certain pump ratings that you must look at besides the pump's physical size to ensure you have selected the correct fountain pump for your needs.
Check the GPH (gallons per hour) rating on the pump. This is how many gallons of water the pump can circulate in one hour. Measure the diameter of the hole at the top of the fountain feature and do a little math. In most cases, for every inch of diameter, the pump will need to circulate 100 gallons of water per hour.
Look for the maximum head height rating. This is how high vertically the pump can pump the water. Water pumped to the maximum height by a given pump doesn't necessarily mean the flow will be very strong at that maximum height. In fact it might be only a trickle. Therefore, choose a pump whose maximum height is higher than the height at which the water emerges from the fountain feature to ensure a strong stream of water.
Check the outlet size of the pump. The outlet size of the pump affects the volume of water being delivered to the fountain feature along with the GPH. Check the hole at the top of the water feature to determine the inside dimension of the flexible plastic tubing. Purchase the correct size pump that offers the outlet size that corresponds with the tubing. If these two things aren't synced, the volume of water the pump is rated for may not be delivered.