Small. I'm taking a stand up fiberglass shower out and expanding it out 3ft or so in one direction with an angled back wall (think pentagon).
Rough numbers, shower is currently 3'x3'. Demo out the back wall and turn it into a pentagon shape with the dimensions of 3'x6'(on its longest side). I would call it 9 linear feet of new wall that needs to be built.
I'm considering taking it on myself as it isn't that big, but it will involve moving one vent pipe and moving the water inlet. I've done a lot of house work before, but never carpentry or plumbing, so I'm a bit timid.