I need to get a new master link how can I tell the size of it? I have lost a piece of my master link for my bike is there a way to find out what size it is so I can order a new one and replace it.
Bike chains come in two sizes.   There's 1/8" chain for single speeds (bikes without a derailler) and 3/16" which is for geared bikes with a rear derailleur.

In addition you can get master links for 6/7/8 speed, or 9 speed or 10 speed or 11 speed.  So count the number of cogs in your rear cassette and go from there.

Master kinks are cheap.  Buy a couple, put a spare on each bike you ride.