The contentpane is not in the text/html module.
The module is in the contentpane.
If you make the contenpane smaller then every module  (including the login) will be displayed smaller.
Waht you can do:
1. Create an extra skin for the Homepage
-Copy the ascx and rename it to home.ascx or something like that.
-Put all the general CSS in skin.css
-Put all that should be differant on the home page in a file called home.css
-Alpply the hoem skin only to the home page

2. You could also reference the Text / HTML module's width with CSS (but the first solution is better)

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