What kind of harness? and how do I measure the size i need?

Last year just messing around I found out my horse will pull a sled in the snow, I made a make shift harness out of a lunge line, so this winter I want ot get an actual harness, so hes more comfortable. So recomandations on what to get, where to get used stuff, and how to measure what size i need would be great. thanks