Unfortunately I don't think it's possible in Darkest Hour.

Thanks for the replies. I was fearing that.

Well, at least the game can use different resolutions, and with a large screen it looks nice enough at 1920x1020, for example. That is better than Europa Universalis, for example.

On a sidenote, newer games by Paradox such as Crusader Kings II or Europa Universalis IV still have the problem of showing very small fonts at large resolutions. Only that in those cases the fan base has provided mods that improve things. But still, Paradox ought to fix that in their new games, eventually.

Since their Steam-only policy, Paradox no longer work for players, but for their shareholders.
But Steam is no longer enough to grow infinitly the earnings, so they sell at GOG now.