For example, your SID file is 28000 x 14000 pixels. Divide that in half to get 14000 x 7000 pixels. Since the largest dimension is still over 8192 pixels, divide it again to get the pixel dimensions of 7000 x 3500 pixels (with a Pixel Reduction of 1/1).
There is currently not a way to change this. If you would like to add your vote for more user control over the maximum pixel dimensions when importing MrSID files, please email email@example.com.
Updated: October 5, 2016