As far as I am aware, at no point in the books is a method of measuring the amount of magical power a wizard has ever mentioned. I found an interesting source after a Google search which has an interesting take on the matter:
The most accomplished wizards in the Potterverse are often described as elderly or hard-working. Voldemort began his reign of terror by the age of 40, but he had spent the previous twenty years working harder than anyone to become powerful. Horace Slughorn was already a teacher at Hogwarts in the late Thirties (and perhaps before), and he is described as an extremely able wizard who evaded Voldemort for one year (HBP4). Dumbledore is 150 years old and has spent most of his life studying. These wizards are powerful because they refined their abilities and amassed an extensive knowledge of magic, and because they had a lot of time to do it.
Thinking about it, I would agree that instead of differing amount of raw power possessed by individual wizards, the thing that differentiates the different levels of ability is being dedicated to improving and having a quick mind to respond to situations (such as in a duel, where fast - thinking is critical). Whether everyone has the ability to use all their magic power (for example, if they are mentally disabled) is a different matter.
If the amount of raw power differed from wizard to wizard, it would seem likely that pure bloods would be far more powerful than half-bloods or especially Muggle-borns. Hermione is often described as being "the best witch in the year", so this seems unlikely.
Like Muggle exams, OWLS and NEWTS are qualifications, and someone who is not very good at magic, but studied hard, would probably do better than a talented one that was complacent and did no revision.