hi i am not an exprt but I have found the same problem ( dnn collapses empty panes and adjusts filled panes to fit all empty space, no pane width parameters in the skin file help to fix this) and solution was to make an image ( call it spacer.gif ) it should be same color as your background and then you place image module ( no container no header for this one as it should be invisible to the user ) in each pane that you don't want to collapse and make the image properties to the required width of the pane.
Lets say you want you right pane to be not less than 180 px, then you place image module to show spacer.gif and set the properties of a spacer.gif to 180px. this will prevent this pane from collapsing less than 180 px .. I hope you got me :) I am not so great of a tutor :) this one sounds hard but it's very easy when you try it!
let me know if I helped :)