I've got a Bike-Packer.
It'll hold a full change of clothes incl winter shoes and jacket. And a laptop.
Leaving the thick jacket and the boots, fitting a towel and stuff won't be a problem.
There's very little drawback from using a big pannier when it isn't packed to the brim.
I have a noname rack which is working well enough.
But if I were to buy a new one, I'd get one with that extra loop by the stays to help keep the pannier stable.