local read_s16 = function(file)
    return string.byte(file:read(1)) * 256 + string.byte(file:read(1))
end

function get_decorationschemsize(f)
    if f:read(4) ~= "MTSM" then error("foobar") end
    f:read(2)
    return read_s16(f), read_s16(f), read_s16(f)
end

fi = io.open("schematic.mts", 'rb')
x, y, z = get_decorationschemsize(fi)
fi.close()