You can if you set the size back to a size that white has.
You basically have a 2x2 matrix with one of the entries invalid, so ebay is not letting that entry be selected.
I'd guess you've got to either make 3 entries white_size_a ecru_size_a ecru_size_b rather than two variations
or
make a variation for white and invalid size but set the quantity available to 0.