This is handled in the HDREDIT-main template form in DEFAULT WWWTPL
You should be able to increase the value in the line

<textarea name=H cols=90 rows=20 wrap=hard>

Look under "Server management interface" and then "Customize site-wide
dynamic web templates" for the HDREDIT-main template form.  Once customized
the customized template will reside in SITE WWWTPL and won't be overwritten
in an upgrade.

Nathan