Condom Size Help.?

Hello Im 14 and just experiment just to find out what condom size i am so if i ever had to have i would be ready and be bale to pratice putting it on and stuff but i dont what size i am. I tried a Medium but it took FOREVER to put it on the ring was to small for the head and was hard putting it on. I am 7.5-8.0 length and 5.5 girth i asked a close friend and he said Magnums but there really expensive and i just think they would be to big. Can somone recommened what size i should get.