Lately I have been using 100 mesh silica and have found this imparts more than enough tooth to the body and doesn't notably change its firing characteristics. It still rings resonantly when struck after firing and displays almost zero porosity. I have used 60 mesh silica and it is excellent but a little more expensive. For a long time I added 65 mesh sand which was quite coarse compared to 60 mesh silica. My current clay fires grey and is very fine.
Fine sand helps it withstand the shock of rewetting.
A white porcelaineous body is more difficult to glaze in some shapes. White stoneware for big pots and hand building. If the glaze doesn't match the shrinkage of the body from the plastic state one of two things will happen. The solution to this is to calcine a portion of the clay until the glaze fits. More usually the glaze will chip or flake off the pot, particularly at edges of rims and handles.