Have you looked at using the attributes controller in the admin panel? We use attributes for various options and once selected and added to the cart it will adjust the price. Sounds like you would need to use text attributes for the length and width unless you created a dropdown list for those. The dynamic updater will update the price on the product info page. I think you would have to custom program the weight update calculation based on the options chosen, or perhaps you can set up individual products for different length and widths and set the weights normally for each one. Not as elegant perhaps but easier(and cheaper if you have to hire someone) to implement.
If you haven't already done so, I would highly highly recommend buying the book eStart Your Web Store With Zen Cart, there is a link in here somewhere. It is a great resource for basic ZC setup including attributes. I found it very helpful and still refer back to it now and again to double check something.
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