Update: Red Xs appear on Google Earth when the computer is unable to render the file at the specified resolution. This is dependent on your computer- we have seen that a Macbook Air is unable to render past 10cm/pixel on larger maps, for example. DD recommends starting with a lower resolution file (50 cm or 20 inches/pixel) and working your way down to the highest resolution that your computer can manage. Another suggestion is to export maps as a set of tiles rather than a single file- you could then break up the rendering into more manageable segments.
Are you on Earth or Earth Pro. I bring in high-res and do a Super Overlay. This breaks the file into tiles similar to what @kara said, but also creates a root (master) KML. This will create a super high-res map overlay.