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.

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Zoom automatically adapts to the smallest dimension, so in landscape mode the role of height and width are reversed.