Assuming you're making a top-down view game, zoom = 5 * block width. E.g. to fit 10 blocks wide, set zoom to 50:
What about height? Well, different users will have different screen sizes, so even with a fixed width the height is variable. But like the image shows, you can compute it by multiplying width by the screen's aspect ratio.
Zoom automatically adapts to the smallest dimension, so in landscape mode the role of height and width are reversed.