Actually, I mostly agree with RT 55J's measurements, but measuring in "map blocks" instead of screens. You can easily tell that the map is comprised of a small number of different segment repeated over and over all of the place. In fact, many 2D games work like this, namely the Sonic series which are often comprised of 128 x 128 or 256 x 256 blocks.
Anyway, visdually the size of the smallest segment seems to indicate a total of aroung 66 x 89 blocks, give or take a few (it's though to tell at that resolution).
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"With the first link, the chain is forged. The first speech censored, the first thought forbidden, the first freedom denied, chains us all irrevocably." [...] The first time any man's freedom is trodden on, we're all damaged. - Captain Jean-Luc Picard
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