Hmmmm, I had the same problem last week. I was after a body armour myself and I wasn't sure which size to go for. Eventually, I ended up in the shop trying different sizes. As i'm 179cm or approximately 5 10, I first tried the  Dainese Impact Jacket Race Suit size Medium. Although I managed to get into it, it was simply too tight for me. There wasn't the freedom of movement that you would want to have as you are cycling. I have a quite broad upper body, so I though I should try the Large one. Initially, I thought it is a bit too big for me. After playing with the straps, however, I managed to adjusted to fit it my body shape. Indeed, it is still slightly loosen on the elbows, yet, I think it is the right size for me. So, my advice is if you can go to the shop, try different things, and then you can order it through wiggle.com for 150 pounds. Bargain!!! I hope this was helpful. Good luck.