I'm guessing that your stitched panorama is too large for the TIFF file format. ICE can only export a TIFF image if the area (width times height) is 536,870,912 pixels or less. That's about 537 megapixels total, or roughly 80 images of 6.7
megapixels each if there's no significant overlap (higher resolution if there is a lot of overlap).
Try reducing the output scale below 100% until the total size drops below 537 megapixels, and the TIFF option should become enabled again. Or you could use another file format, of course.
Where does the limit come from? The TIFF file format uses 32-bit offsets internally, but some implementations use signed integers, meaning it's only safe to use 31-bit offsets. This effectively limits the maximum file size of a TIFF image to
2^31 bytes, which in turn limits the image size to 2^29 = 536,870,912 pixels in area, assuming four bytes per pixel and no compression.
Eric Stollnitz, Computational Photography Group, Microsoft Research
