The basic space you need is the table to hold the enlarger and three processing trays. If you have a separate wet area and it is dark, you just need the darkroom to hold the enlarger. It can be 4' x 5'?
For people with very limited space, a kitchen, laundry, bath or mud room is a wonderful place to house the processing equipment, be BW processing trays or a Jobo for color processing. You can put them away in closet when you do not use them.
A photo amateur
Sinar P2/F2/Nikon F100/Bronica ETRSi/GS/Saunders 4550XLG/Jobo CPP2/CPE+/Colorline 7000
My 'dark area' is in a room about 6' x 12'. However, the actual space I have for printing is just enough worktop space for the enlarger and three trays as the room is used for other things as well. The running water is in the bathroom which is the next room along.
"People who say things won't work are a dime a dozen. People who figure out how to make things work are worth a fortune" - Dave Rat.
I don't really know, can only guess. I left there in 1987. Probably 4 feet at the most. (Look at the picture) It was essentially a 10-12 ft. kitchen counter with a single bowl kitchen sink right in the middle. As can be seen in the picture, the enlarger (a medium format Phillips) hangs off the edge of the counter. I had more space not seen in the picture for storage, but it did not contribute to "darkroom" operations.