Trimming down units
Why is it patterns that I have read over the years they don't indicate the finished size of a unit in a block. For example a flying geese unit. I've. Heard and read many quilters will make the unit a bit bigger than indicated then trim it down. But I'm hesitant at times on what to trim it down to as I may figure it out wrong and it will not fit with the attaching unit. Since I'm making all the units at once then go to the next part of the pattern and make that unit. So what have you all found to be the easiest and accurate way to accomplish the "trimming down" part.
Hope I'm explaining my question clearly.
Thanks for your help