Are we to assume Tub Wall or Tub Floor ? You mention Bull Nose is why I am asking.
There are not many standards but too large will look odd - I would do a MAX of a 12 inch for the floor (I have this now and it is fine) - but I would also make it a Diagonal layout.
6 or 8 Inch tile would probably be best for that size.
3/16th Grout line - easier to lay 1/8 inch much harder ..as small height changes tile to tile are more noticeable..
Get a nice grout color that blends well.
Spend the extra $$ for the Water proof board.. you can also lay in the rubber seal mat . Especially if doing a shower wall ..
Shower / Tub wall:
For a wall you should use both the durorock and also the waterproof Tile Membrane - while you can do this without the Tile Membrane - I suggest to use it.
The 3/16 inch grout line is really wide for a shower / tub wall 1/8 is better, personally I would do no more than 1/4 ..
As for tile size depends on how big your wall is and what look you want to achieve. Some people use subway tile (4 or 6 inch) b/c they 'love it' . I hate that stuff.
You can use 8 or 12 inch very nicely - measure out your wall make your design on paper - KNOW where it will start and end on the back wall, the sides and and near the ceiling. So you will get the proper layout. Start in the center of the Back wall so you can split the size difference - in case you do not have a perfect fit of tiles plus grout.
REMEMBER TO MEASURE YOUR TILE
1: Your tile might not be exactly 12 Inches it might be 11 and 3/4 inch. That way you can use that as a measure number when determining how it will lay out on your wall. Tile Width + Grout Width .. / or Tile Height + Grout Width
2: If you are going to run the tile to the ceiling - remember that you will have difficulty with any crown molding that you might have in the bathroom .
3: If the tiles do not go to the ceiling - you will have a gap and might need to use bull nose around the top $$$$$ .. running all the way up might be cheaper.
4: Also if you want a soap tray for a shower in the wall - Plan for that in your design - it is not hard to do since you are making the wall .. just remember to put a slight downward slope towards the tub on the base tile - use a small piece of extra Tile Membrane on the back side (1/4 inch strip) of the base area to give you the required slope.