I've heard the way to measure grip size is grip the racquet handle and then lay the index finger from your other hand in the gap. If there is no gap, the grip is too small. If it kinda fits, then its a good enough size. If the gap is significantly larger than your finger then its too big.
However there is no science to this. Some people have different preferences that match the way they grip the racquet. I know a guy with smaller hands than I (uses a 4 3/8ths) who loves a 4 5/8ths grip
In your case with pain in the ring and pinky, if it's a numbing and occasionally throbbing pain it could be related to nerves around the metatarsals (and maybe down to the carpels and the wrist) with the way you are gripping the racquet. Could be overdone and the muscles in the palm have tightened up from overuse or strain. Consider massaging the area, stretching wrist and fingers and maybe building grip strength.