possibleValues = [-1, 1];
A = possibleValues(randi(numel(possibleValues), desiredSize))
This assumes you want -1 and 1 to be equally likely. If you need an exact number of each possible value:
elements = [repmat(-1, 1, 5), repmat(1, 1, 25)];
order = randperm(numel(elements));
shuffled = reshape(elements(order), desiredSize)
If you need the two numbers not to be equally likely (say 1 three times as likely as -1) there are ways to do this as well.