Please help me in deciding what size subway tile size and grout to use in my kitchen project.

What is the ideal size, 3x6 or 2x4, to use in my kitchen? Will the 2x4 look dated in the foreseeable future as it is not the typical size?

What color grout? White? I'm afraid grey will look choppy and compete with the floor which has grey grout.

I am using a meram blanc marble subway tile for the backsplash with Caesarstone Piatra Grey counters and white cabinets. Backsplash is only 12" high from counter to cabinet. The floor is a white ceramic with grey grout. Kitchen is small, appx. 12'x15'.

Thanks!