What you failed to indicate is that Sis is a non-scalar struct. Sis.D is then "struct expansion". So
size(Sis.D)
is the same as if you had written
size(Sis(1).D, Sis(2).D, Sis(3).D, Sis(4).D, up to Sis(63).D)) %or whatever the size of Sis is
You can inquire about size(Sis(1).D) or size(Sis(end).D) or as appropriate.
If you need to know the size of each of the Sis.D entries then
SisSizes = arrayfun(@(S) size(S.D), Sis, 'uniform', 0);
If all of the Sis.D entries had the same dimension then you could also proceed to
SisSizeArray = cell2mat(SisSizes(:));
which would be a numeric array with numel(Sis) rows and numdim(Sis(1).D) columns
