i have a skin file which has two basic columns.  a cell on the left. the leftpane with a menu in it and the main contentpane cell filling up the rest of the page. 

I have inserted a forum module into this pane but it will not fill up the cell width.  the forum is restrained to fill up the cell.  the cell goes to the edge but the module will not. 

What actually controls a modules width?

I might add it is working in firefox using the web developer tool that lee recommends and is the best tool for figuring this out. but the forum is not filling the cell in i.e.