You have to pick:

Cell duplication won't change the size of my bones, since bones are mostly composed of extracellular matrix material.

My brain just increased its size by a factor of 1.5 inside that same-sized skull, though.

I imagine the effect would be rather gruesomely explosive. Good thing it'll be painless, since all those extra brain cells probably instantly disrupt any conscious mental functioning.

@derksenmobile Why do you always have to ruin everything! :'(

Does this only compress your height?
After 1000 sneezes you'll be 36.77% of your original height. You'd start becoming very oddly-shaped if it didn't shrink all your dimensions.
Do the WYR Pixies magically make it possible for your body to function at extremely small sizes?

Your body stays at the same proportions and yes, the WYR pixies make you able to survive at tiny sizes.