All field keywords beginning with
html_form_ are passed directly into the form hash.
they are really defined in form.tt which ships with Gantry.
html_form_rows specifies the height for fields of type textarea.
The value is used literally in the rows attribute for the textarea HTML tag.
To see an example, build:
bigtop -c example.bigtop all
Change to the newly created Kids directory and look in the form method in lib/Kids/GEN/Child.pm.