You know those 18-gallon Rubbermaid bins with the lids? You can put six of those in the thing with the rear seats down, and have plenty of room around them for things things that are flexible or small.
Never drove a Fit. I'm 6"3, north of 300 lbs, and I'm comfortable with the driving seat on the third stop forward of the limit. I've never owned a car that I didn't have the seat all the way back before.