Is there a standard or guideline to this? For instance, is there a prototypical person with an average weight they design dosage parameters for?
It varies; however in general it is based on a healthy individual ages 4 - 50 consuming 2,000 calories a day.
The LiveStrong web site states:
The daily dose of vitamins health officials recommend varies by life stage. These groups are defined by people's gender, age and, in the case of females, whether they are pregnant or lactating. To maintain body metabolism, everyone needs the essential vitamins A, C, D, E and K and the eight vitamins in the B family. The recommended daily value printed on vitamin supplement labels is based on a 2,000-calorie diet and represents the average vitamin intake adequate for those age 4 to 50. The U.S. Department of Agriculture (USDA) recommendations are higher, however, and represent optimum doses.
There reference to the USDA allows us to get a much more granular view and covers the varying life stage groups.
If you take a life stage group and what the average weight is for that given group, that would define your prototypical person within that category that they base their figures on.