You'll need to use the Scale Recipe function to vary the amount of ingredients for various batch sizes.
As you use BeerSmith, you'll find that it accounts for a lot of variables. So, when using a recipe source, you need to make sure that you have their equipment specifications in place for losses, kettle volume and a few others. Then when you scale the recipe, you'll tell BeerSmith to use your equipment so that the recipe will create the same gravity, bittering and color as the original.