Using Product Configurator you can do that as a product option.
Define the scenario in the main admin panel at left Product Configurator Options => Manage/Define Configurator Fields. Then set them in Configure Products => Configure Instant Pricing and Product Configurator. In Manage Pricing Structure Options => PRICING ADD-ONS BASED ON PRODUCT CONFIGURATOR VALUES you choose to make the options required or not. And finally . . . in the MATRIX FOR W x H button = set the price at -999 if you need it hidden for some sizes and 0.00 if you need it shown for some sizes. All that will make them show in the cart in the product as options where user will make a choice. If set up properly, user cannot move forward to submittal without making a choice.
This is all possible because of the fix in this conversation
in the forum. It appears no one ever followed up the conversation to report that the fix was done however you can now successfully stop user from moving forward until a choice is made. Works great.
I hope I got all that right. I doubt it, but with a little investigating I'm sure you can get it set.
