I think having a little more space will be more comfortable in the end but a smaller tent will be better for warmth to a point. I think a 1-2 won't be as big as a 1-3. If you have a lot of open space in the tent you will be colder but if you have the space filed either by yourself and gear or you and a friend you will be in better shape.
In terms of keeping warm:
- make sure you have good moisture wicking breathable layers (no cotton)
- keep your head warm
- leave space in between those layers (so no super tight stuff). Just adding more layers won't help if there is no air to trap the warmth. Throwing on a jacket in your tighter sleeping bag won't necessarily be warmer just because both are warm,
- have a good sleeping PAD to insulate you from the ground (the higher the R value the warmer it will be)
- know that sleeping bag ratings are all over the place sometimes but generally expect something at the temp rating it is outside will be tolerable but might not be comfortable.
- Sea to Summit makes various sleeping bag liners including thermal ones to up the temp rating there might be others but StS makes really great stuff that I use and reccomend
- keep your head warm (not a typo I wrote it twice because people forget this and you lose a lot of heat through the head)
If you are van supported then take some extra stuff, If you don't need it maybe someone else might find it useful. If you are the type to get cold have a water bottle that you trust to be leak proof and fill it full of hot water and keep it in your bag.
If you really really get cold a 4 season mountaineering tent works nicely but you need to vent it or you will get condensation but I used a 2+lots of gear North Face solo in some snow near a stream and was quite warm without the benefit of the stove that others had. It was quite amazing. However is a heavy tent and very much overkill for most things. You could also add in a cot with a pad on top that is generally what I do for short trips or supported rides when carrying way too much crap doesn't matter as much. You and your pad are off the ground and more comfortable but most of that is generally my bad back.