Think about the proportions. Your boxes are built around what looks like a 16x16 scale that is blown up.
Your sprite is 16x18. If you blow him up like the boxes he will look fat and square(boxy

)
Since you are rendering this all into a 3D landscape you need to aim closer to reality to capture realism. This means that you should have your sprites be much taller than they are wide. Almost all human beings are somewhere between a 3:8 and 4:8 ratio for width height (shoulders width.)
You should aim for something like that maybe.
I would personally use a factor of 2 on your ratios and work with 24x24 or 32x32 sprite restrictions. It works well with pixel proportions and you can capture your ratios easily with it.