For the size of polytunnel and/or greenhouse, just in my experience whatever you have will never be quite enough – so I’d say go for the biggest size you can fit and afford. I know that sounds a bit vague, but once you start growing and exploring possibilities the seedlings expand in number in inverse proportions to the planting out space available!
As for what to grow – perhaps start with what you already enjoy eating, plus a couple of things you normally wouldn’t. Veg tastes very different when home grown, so things you might normally dislike (beetroot and brussels sprouts spring to mind) are actually very tasty.
The climate in Charente is pretty good isn’t it? You should be able to grow lots of wonderful stuff, good luck!