I remember when I was a gel that wellies always had to be bought a size too big. I don't know if this was to allow for growth or ease of putting on and taking off or if wellies just notoriously always came a bit smaller than their size.
I haven't had any for ages but I would still get them too big these days so that if necessary I could wear 2 pairs of socks under them. Good tread is vital for us "frail" old biddies, too.
I hope you could exchange them for a better fit!
Static caravan purchase is it worth buying