Hypothetically, what is the smallest functional form of human?
This question is for Doctor Frankenstein!

The answer would depend on what you would accept to consider being a human.


Does it still need to be able to make simple calculation in order to be a human?
Yes, because otherwise we would have lost some functions. But then you may ask…
Does it need to have big enough hands in order to fulfill the social function of hand shaking.
Does it has to be attractive to the other sex?
Does it need a heart or is it fine if oxygen diffusion through tissue only is enough for this creature to live?


I guess that if you change the number of cells you will necessarily change the functionality. I am not saying that small guys are not as functional as big guys. For example, big guys are better runner, while small guys are better climber. 


Are we allowed to shrink the cells to have minimal sized cells?
Are we allowed to shorten the DNA in order to shrink the cells?




Notes:


There are approximatively 200 different cell types in a human


My reference is not a peer-reviewed article (ref). Wikipedia lists cell types and it doesn't seem to go over 200. However, it is important to realize that it probably very much depend on how you define cell types.

Terminologica Anatomica lists over 7,500 names parts of the human body
There are 13 or 14 (depending on the gender) gland types
This article provides an estimation of the number of cells in the human body 
Here (wikipedia) is a discussion on the concept of the minimal cell
Here and here (biology.SE) are discussions on the minimum number of genes/amino acid to sustain life
Here (biology.SE) is a discussion on which species has been found to have the smallest genome