The length of the listing description can be set in Configuration> Product Listing > Display Product Description.

HTML is disabled in the listing because truncated text might cut off the end of a tag and mess up the display. If you set the display longer than you ever expect your descriptions to be, you can safely re-enable HTML using the directions in this thread.

You can also make different parts of your descrpition display in the listing page and the info page, to keep the length down in the listing, as described here.

You can get "handles" for the elements of each product listing as discussed in the Add CSS Classes for Table Columns thread.