You can set the static block width.
In order to set your own fixed width for static block please use the following syntax:
wp_custom_menu_15_w320
where w320 is a directive to set static blocks width to 320 pixels,
15 is the ID of the categoriy.
You can hide empty categories.
Please open Backend->System->Configuration->WebAndPeople->Custom Menu section and set "Display empty categories" field to "No" value.
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