I find them pretty true to size. I have size 4 and I could be anything from 3 1/2 - 4 1/2. They fit with any socks I put under, including my thick woolies, so if anything they have a decent amount of room.
I ask because the styles are vastly different imo.
I'm normally a 7 or 8 (mens 7, womens 8, 41/42) I got a pair of Argyll mens in a 7, really large on me Then a pair of ladies SArgyll in an 8, again too big
Finally bought a pair of the originals in our local scats, they are perfect and they're an 8. they are hard to get off though as the bridge bit over your foot is quite trim, I wore them sockless once and they really got STUCK, ds had to try and pull them off
I have the Huntress style, which are wider in the calf. They come up large but not extraordinarily so - my true shoe size is probably 5.5, normally 6 in 'fashion' stuff like Next, 5 in Dr Martens, and my wellies are a 5. Just a shame that the gift of calf don't get a very interesting selection of colours.