Best Radiator for Combi boiler

We are replacing our old radiators for new ones and also the combi boiler for a new boiler.

We have large windows in all bedrooms but rooms are not too large. We have zero experience with radiators and were wondering.

What is best single or double radiator?

If we can go up to 1200mmx400mm radiator would that be enough for a 16sqm room?

As the rooms don’t have a lot of space would it make a huge difference single vs double radiator?