“Tomato cage” plastic connections — Design idea

Most builders never even consider this and just create a building large enough that some waste can be reused (e.g. small pieces of drywall in a closet or a bad piece of lumber as a cripple below a window) while most waste is just tossed.

For something you build yourself, start with the outside measurements and get those on an even measurement. Consider how the wood sheathing will overlap on the outside, so an 8' span of 1/2" material can actually cover 8' 1/2" since one corner will have the next piece overlapping the end. Don't worry about cutting an inch or so off the floor, since as you say, you're better off removing material than cutting a patch from a larger piece. Studs come in standard heights, so just use that height.

When it comes to picking the thickness of lumber, I'm a firm believer in overbuilding the floor of the shed. It costs a lot less to overbuild this today than to rip things apart because the joists have weathered poorly. For the rest of it, I suggest leaving it exposed as a shed. The ceiling joists make a great storage space, and it's nice to nail things on the walls, so interior finishing of a shed is a waste of material IMO.

For things like siding, doors, or other less generic materials, see if the local Habitat ReStore has anything useful in stock. Most of these are waste products from a contractor that bought too much or the wrong style, so you get to help a charity and keep things out of a landfill at the same time.

You can also look at Wet Zones. These come in a variety of styles but the one we used from Aqua Dry consists of a raised base you fix above the existing floor level.

This base has the correct drainage slopes built in - all you have to do is make sure the floor on which it stands is level. The range includes side and end panels which fit onto this raised area.

The arrangement we have is (approximately):

The shower head (not included in the kit) is virtually directly over the drain and points more or less straight down. We don't get any water outside the shower area beyond a few splashes.

The walls still need to be made waterproof as does the junction between the walls and the base - but the kit comes with all the tape, glue and waterproofing needed. All you need to do is add the shower itself and the tiles.