This is about aspect ratio, not size. Many cropping tools, however, lead one to belive you select inches or cm or whatever. You really only need concern yourself with ratio, so a 4x6 if can be an 8x12.
I shot slide for many years, about 20. I was very concious of filling the frame. With digital, I had to retrain myself to leave some space just for the chance I want a non 2x3 ratio.
But, it really doesn't matter much, if you play around with and image and try different ratios (Square for example) you will find many hidden images within one image, in other words the shot you composed in the field can generally be recomposed in various ways after the shot.
I wouldn't sweat it. You have great gear and know your images show you know what you are doing, this is a surprising question in light of that.