How can I stop internal wooden doors expanding in the winter?

I know of no treatment which prevents wood expanding and contracting. Real wood expands and contracts, always has, always will. The construction of most doors means that this isn't usually a problem but you do need to make sure you leave sufficient clearance around the door to avoid issues. Sealing the door may reduce the amount of moisture the wood can take on and therefor expand so painting it with an oil based paint. If you do this, make sure you seal the whole door, including top and bottom edges but you'll never stop it expanding completely. If you like bare wood (and if I were you) I'd just take the doors off theirs hinges next winter and shave off the offending areas. I've not known a wood door to expand / contract by more than 3/4 mm so that shouldn't leave you with too big a gap in summer. Alternatively you can go for a Real wood laminate door which won't expand!