The sizing is to go over the riding jacket - so if you have a medium riding jacket, pick a medium one of these. It has nice long arms, well tailored, and decent cuffs which fit over your heavy gloves. I have the Alpinestars AMT-8, which although it comes with a rain jacket, it's black, and I wanted something more visible, so this was perfect. It comes with a bag made of the same material to store it in. I haven't tested its waterproofness yet, but it rates highly, so I'm confident it'll be good.