"Doing this for the structured array returns 1 where it should be 4."
No, it should return 1 because you have a scalar structure, you even wrote this yourself: "I have a structured array called 'Data' which is a 1x1 struct". A 1x1 array is scalar, i.e. has only one element, regardless of its class:
In contrast a structure array with four elements has ... four elements:
S = struct('X',{1,2,3,4}) % 1x4, not 1x1
length(S)
The size of a structure array is completely independent of how many fields it has.
