There is nothing very scientific about measuring a blister; measure it just as you would a mole.
With a ruler held above or next to it, note it's length and it's width, and it's general shape.
Blisters from burns or friction or do not tend to "grow" once the damage to the epidermis is finished (The skin is cooled down or the shoe comes off). They may become more tense, but that doesn't change their length and width.
Blisters from a process that separates the epidermis in an ongoing manner (e.g. an autoimmune reaction such as bullous pemphigoid or a drug reaction) or from a slow-acting contact irritant (e.g. poison ivy), etc., may grow.
If you have blisters from an unknown source, you should see a physician.