usually grout line depend on the size of the tile, larger the tile larger the grout line up to about 1/4 of an inch

I like to do alot of things myself but installing granite to the professionals

1/4 inch is what you will have at least, I have seen grout less but it was in a living room.

Grout is required when you put tile, of any type, on the floor and walk on or place pressure on the floor. The grout actually allows the tile to give at the grout seams... since you are not walking on the wall granite, you do not need any grout. you can caulk the seams..

Granite can have small grout joints, you can go 1/8 inch if you want

I would butt them up together and push in some sandless grout. I don't like big grout lines either. Caulking seems would be very tedious and need to be replaced.

I have done granite on many vanities and counter using groutless installs. For showers I have installed 1/8" to 1/4"...My preferred size in 3/16 with sanded grout...sanded grout at this width is very durable. Unsanded grouts upper limit is about 1/8" beyond this in can tend to crack.

Contrasting grout can add some design elements...look at the black grout in my article on back splashes...The last pic with the black granite shelf. ...»

I would also suggest looking on line for a solid slabsof granite

around the shower. their several companies that offer shower surrounds with a i/2 inch or 3/8 inch thickness. slab that will custom fit the shower and there be no grout lines.