I don't think it much matters where I measure it because I'm not going to stay hard while I try to get a tape measure up against it ;p
But I think I would say the side, just because then the angle (assuming only up or down) will affect it as much.
For me the only way to measure mine is in a pussy. The truest measure is in oohs and awes.
To me the issue is not so much how big a man is as how beautiful his male member is
I usually measure cock with my hands, if I need two hands to hold it then that is great. Then with my mouth, if I can get it in my mouth without too much stretching fantastic, then finally with my sex. If he fits me, fills me but I can still feel his balls hitting my arse when he is fucking then he is the perfect size!!