The number of rows for textarea for the description of the taxonomy term by default is 5, but in category module taxonomy term is a node and it is not enough for work with node text.

At editing with the WySWIG editor there is no possibility to change allowed textarea height, and 5 rows it is not enough height.

In category.module should be 

$form['body_field']['body']['#rows'] = 20; //increase textarea height from 5