There should be enough space around the toilet and sink for comfortable use: about 30 in wide and 36 to 40 in in front. You need at least 8 in on either side of the sink.
Allow at least 30 in in front of the bath for ease in climbing in and out of the tub and the same in front of the shower. This space also will let you towel off without hitting the walls.
A separate shower stall should be at least 42 in in one direction (length or width).
Countertops should be 32 in off the floor for a normal-height person; a tall person (more than 6 feet tall) should have a 34-inch-high counter.
The bottom of the mirror above the sink should be set 40 in above the floor.
Fixtures and fittings are usually located at these heights in standard bathrooms.