I am running a feeder to a panel in my outbuilding. Do I use my calculated load or do I use the 100 amp main breaker rating for my voltage drop and feeder sizing calculations?

The voltage drop calculation is determined by wire size and length of run. How far are you going?

I am running nearly 190 feet to the outbuilding. Planning on a 100 amp double pole main breaker although the calculated load is significantly less.

Do I have to gauge my wire on the calculated load or the 100 breaker rating ,as well as figuring in the voltage drop?

Base your calc's on the feeder breaker.

As an example, you are allowed a 60 A feeder and a 100 A subpanel main in your outbuilding . That 100A breaker will act as a disconnect.

The wire size is based on the feeder breaker size and distance.

Karmalcopia, as Kiss said...
Here's a calculator you can use if you like.
Wire Size Calculator