Code
%%// Get all the arrays/vectors into a cell array, with each element in each cell
A = [{a},{b},{c},{d}]
%%// Get the number of elements of all the cells, that is number of elements
%%// of all vectors into an array. This, this array could be thought of as an
%%// array of sizes
numels = cellfun(@numel,A)
%%// Now that we have an array of sizes, we need to make sure that
%%//all the sizes are same. So when we compare all sizes to
%%// any randomly picked one (first one in this case), must be matching,
%%// which is being covered by the command - all.
if ~all(numels==numels(1))
error ('Length of data series is unequal. Correct it.')
end