There is no one size - the reason you get square footage answers is because there a lot of various dimensions one could use to achieve the needed amount of space. Are you wanting a square, rectangular or round brooder? Each would have different dimensions to achieve the amount of square footage inside that would be needed for your birds.
ETA - the size would also depend on how long you intend to keep the chicks in this brooder. The space needed by 10 RIR chicks at 2 weeks will be quite different than the space needed if you intend to keep them in there to 6+ weeks.
Sorry there is no simple answer to that. Some people take the chicks out of the brooder before five weeks, some wait until 10 weeks or even longer. They grow really fast. Cockerels require more space than pullets. Is the brooder in the house, in an outbuilding like a garage, or in another outbuilding? My brooder is in the coop to start with. A lot depends on how you manage them. I’ll make some suggestions but there is no magic size that fits every brooder in the world. There are a lot of different factors.
For 10 chicks kept until they are 5 weeks old, I’d suggest a minimum size of 2-1/2 feet x 4 feet. This is probably bigger than the absolute minimum you will need but there are a lot of unknowns. If you are going to keep them in there until they are ten weeks maybe a 4’ x 8’. I’ve never kept them in the brooder until they are that old so there is some guesswork there but I think you will be OK. It may be crowded, especially if it is mostly males.
If these are your first chicks and you have power to your coop, raise them in the coop as long as it has predator protection and protection from the environment. Just provide heat in one corner so they can go there when they need to warm up and they can self-regulate when they need to cool off. That will avoid a brooder altogether.