Splitting Rent ... what do you think is fair

So two scenarios. Let's assume attached bathroom for each room so that's not effecting your answer.

1. House has 3 rooms of the same size. rooms 1 and 2 have a single person renting it. Room 3, a couple is renting it.

2. House has 3 rooms. Two rooms are the same size, the third is larger (aka the master). Two rooms have a single person renting it. While the last room is occupied by the couple. (No information on who is renting the master).

Now in these scenarios. Should rent be spilt equally among all who live there, 4 ways? Or should rent be split by the square foot of the room, so regardless if you are a single person or couple, the rent for the room is only more if its larger then the other rooms?