You have several options here. Probably the simplest is to return a cell array and then concatenate the contents. It might not be terribly efficient though if N is large compared to the number of outputs you're returning (this is because cell arrays where each cell contains only a small number of elements have memory and performance overhead compared to a numeric array).
Here's an example. (Note the use of feval to work around a PARFOR restriction on using function handles)
% Here's a simple function that generates a random array of a random size:
generate_result = @(x) x * rand(1, randi([0,10]))
parfor idx = 1:7
output{idx} = feval(generate_result, idx);
end
% Now, concatenate the output elements into a single array
output = [output{:}];
