Size, somewhere between 3 and 3.5 inches of blade.
Make sure you buy one that is flat ground the whole hieght of the blade, or darn close to it. This will give you the best cutting.
There are some different blade shapes to consider. Most common in the cheapies is a clip point.The last inch or so of the top of the blade is clipped in a straight line to the point. For the fine work of paring blade, that's too steep of an angle in my opinion.
Better is the drop point that drops in a smooth gradual curve along the spine of the blade to the point. This gives you a fine point for delicate work.
The cutting edge should be mostly straight without lots of curve at the tip. Some curve is usually desirable.
Another shape, the one I prefer is the Wharncliffe. Jaques Pepin can be seen using this style sometimes. This style has a straight blade and the spine curves to meet it at the point. I actually use a folding pocket knife one. It has a clip on the other side so it attaches right in the pocket of my apron from use as needed.