For cold nights I double up with a cheap light sleeping bag over my regular 3-season bag. The difference is huge!
In Europe I buy this one from Decathlon.

As to tent size, I agree with others that a tent big enough to sit up in and get all your gear into is preferable in inclement weather.
I’ve got a Hilleberg Soulu, which is officially 1,5 persons tent.

Oh, and IME sleeping bag ratings are usually incredibly optimistic… stupidly, even dangerously optimistic if I’m being honest.
Some people sleep warm, others need more insulation, so ymmv