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an integer is 2 bytes. You're going to want to use that as your array type.
Now that you've established how big one element is, think of how many bytes per pixel. In screen 13, this is 1 byte. In any 16 color mode, this is .5 bytes. So after that, you're going want to figure out hte number of pixels (width * height), and divide that by the number of pixels in each element. (and then subtract by 1 since arrays start at zero by default)
dim tilevga (width * height / 2 - 1) as integer
'any 16-color mode:
dim tileega (width * height / 4 - 1) as integer