It is much easier to perform these kind of operations when all of the data are in one variable. So that is the first thing to do: lets put them all into one cell array (this is perhaps how they should have been saved anyway):
>> A = randi(9,1,5);
>> B = randi(9,1,8);
>> C = randi(9,1,9);
>> D = {A,B,C}; % put into a cell array
Now finding the array with the most elements is trivial:
>> N = cellfun(@numel,D)
N =
5 8 9
>> [~,X] = max(N); % its index
>> D{X} % extract that array
ans =
5 7 7 6 1 1 3 5 6
