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.