First of all, you should determine whether you need prescription swimming goggles to help you see better. If you have perfect vision, then you just get non prescription swimming goggles. Otherwise, you should get prescription swimming goggles. Next, determine if you often swimm outdoors. If so, you need swimming goggles with UV protection and mirrored or tinted lenses to protect your eyes from the glare and UV rays. What's more, you'd better choose anti fog swimming goggles to prevent the lenses from fogging up.
The best way of choosing the right size swimming goggles is trying on to find the best suitable one. When you try on, put the goggles against your face and press down lightly to test the seal. If you feel a second of quick suction when you press, that should be a good pair of swimming goggles for the right sizes. Or it may not protect your eyes well when you swimming because they will likely leak.