They def come up small, I am 8-10 and wear medium with NF.
I like this jacket and thought about buying it for a while, but I can't excuse another waterproof purchase.
I have always found them to be adequately waterproof, more so than many regular brands such as Joules, fat face, etc.
Remember this is a shell, so will have a slightly different but thicker texture than the usual waterproof fabric. Amazing for keeping the wind off, and whilst not at the top of the waterproof range it will be just fine unless you are standing on top of Ben Nevis for 5 hours in a freezing rainstorm with nothing but a backpack and flask.
The design looks good for repelling water, ie the cuffs, hood, zipper.