There are three simple approaches for estimating. They are described by Mike Cohn in his Agile Estimating and Planning book. They are very common used but worth mentioning once again.
First way is to ask an expert. Expert relies on gut feeling and experience. It is hard to find proper cross-discpline experts for software development but it is still not so bad way to go. Benefit from this kind of estimation is fact that it is usally very quick. Expert read document, ask question or two and already have something in his mind.
Simple variation which I personally often use is to ask many experts the same question and make your own conclusions based on their estimates.
It is simple technique. We stop asking “How big is it?” and start asking “Is it bigger than that?”. It is fact that human are better in estimating relative size instead of absolute. Just use Scrum Team estimation game
It it really hard to estimate if very expanded User Story is 10 times bigger than other of 20 times bigger. In such cases we always try to split bigger thing into parts which are easier to compare to others.