It is 64x64 pixels per block, so you have to multiply it with the blocksize you want.
So 64 x 3
And 64 x 4
So it indeed should be 256 by 192.
But it depends what you are spriting. Walls should be the whole size, including the floor. But if you make a turret, it foesnt have to be that size. Just like doodads.
Could you explain what you are spriting?