How do you rough in the plumbing for a bathroom before the slab is poured? In particular, the sewer line and the drains for the shower and sink. How deep in the ground do these pipes have to be? What size pipes are customary?

Depth will depend on where you start from.. but generally I keep my pipes about 3 inches below the dirt as a minimum.

All pipes 3" and smaller should pitch at 1/4" pitch per foot of pipe. Minimum size pipe underground is 2"...even for vents.

All changes in direction from horizontal to horizontal pipe should be made using a long sweep elbow or by making a long sweep elbow from a 45 degree fitting cemented to a street 45 degree fitting.

Toilet pipe size is 3" minimum
Shower pipe is 2" minimum
Sink is 2" minimum
Tub, underground is 2" minimum...reduced to 1.5" after coming out of a 2" ptrap

Vent for lavatory should be 2"

Most bathrooms in my area work like this:

Pipe 3" to toilet, take a 3"x2" wye off that 3" pipe (just before toilet elbow) and pick up tub/shower and lavatory. The sink gets picked up above floor and 2" vent from this sink acts as a wet vent for the whole bathroom. This vent needs to connect into vent at least 2" in size which may be found in basement or upstairs in attic.

Check out the top picture below... only thing not correct in picture is that picture does not show connections connecting via WYE fittings. You must use wye fittings underground here! Also, in the top pic. There is large vertical pipe that is required if this is your only bathroom... so ignore in this case because you don't need it.

Wet venting may not be allowed in your area so need to check on that (see line drawing). I drew using wye fittings.

If not allowed then the tub/shower gets its own vent, toilet gets its own vent and the sink also gets its own vent (see drawing of individual vents).

That should get you started... let me know any questions that you may have... glad to help.

MARK