It depends on how you arrange it, but 5' x 8' is as small as I can think of quickly.

I've seen 6 X 6.

Is the washer and dryer in the shower?

The classic cheap apartment bathroom is 5 x 7, all the plumbing on one wall. First inside the door is the sink, then toilet and tub all the way across the end wall. If you widened that to about 6 feet, you could substitute a 42 inch shower pan and then have enough room left over for a stackable W/D unit

A more decent sized bathroom/laundry room would be 5x10.