Hello thoinghidi,
You have Split in Blocks option checked on, so the orthomosaic will be exported as 1024x1024 tiles.
However, it seems that with the current total size of the mosaic it may be problematic to export the image in the single TIFF file, since this format is limited by 4 GB size. So you may need to increase the size of the tiles by 10-15 times by each side, for example.