there are basically 2 sizes... 2.5 mm and 3.5 mm when it comes to the correct fitting jack connector that plugs into your radio or listening device. I own a few of both sizes... some are for my Motorola 2 way portable radios and others I use for my Pro-106 scanner. They are relitivly low cost, around 10 to 20 bucks on ebay, which is where I purchased the majority of the ones I currently own. They work well, and look cool. Good luck to you, I hope this info helped you, and enjoy this RadioReference site my friend !
ps, keep posting if you need help... there are no dumb questions here, and many members will be glad to assist you when they can.