The preview pane on the Import Wizard window tries to show all the elements of selected matrix which may fail if it has too many elements. It seems like the maximum number of elements that the preview pane can show depends on the dimension of matrix, because the 1-D matrix show below has no problem to be previewed.
>> a = rand(20,20,10);
>> a = a(:);
>> save('a.mat', 'a');
Hope this helps.
