How to use formencode for textarea with multi rows

Question

my code:

name = validators.UnicodeString(not_empty=True)
slug = validators.UnicodeString()
condition = validators.OneOf(ItemConditionList(), missing=None)
desc = validators.UnicodeString(input_tag="textarea")

my web: https://i.stack.imgur.com/OlFP8.png

my question: I want the "Desc" field with default 20 rows. I try with formencode 's document but i can't. Please help me! Thank you!

UPDATE Sorry, my question perhaps because i use package "pyramid_auto_hq" (auth: tarzanjw)at: https://github.com/tarzanjw/pyramid_auto_hq/blob/master/lib/bootstrap_renderer/horizontal/smart_input_row.py

what should i do? contact to Mr.Taranjw or any ideas?