This is a great question for our support team as well. There are a couple of ways you can go about this. You can add a between-concept prohibition by clicking on the Prohibitions button in your CBC attributes tab. Then click on the Advanced Prohibitions tab. You can technically enter the prohibitions in the tab here by hand using the numbered attributes and levels that you do not want to appear together. I like to use the Add Prohibition button to help me out.
Click on the Between-Concept radio button and enter how many concepts you want to prohibit in a certain scenario. You do not need to select a level for each attribute. Depending what version of the software you are using, alternative-specific designs may not work in this window. If you have a lot of combinations between size and price, you can also set this up in Excel and paste the prohibitions in the table view.
Do all of your prices and sizes appear together? I always recommend testing the CBC design before fielding. Between concept prohibitions are not the most harmful type of prohibition but a lot of prohibitions can still hurt the efficiency of your design. If you have different pricing tiers for the sizes, you can consider using conditional pricing (http://www.sawtoothsoftware.com/help/lighthouse-studio/manual/index.html?hid_web_cbc_cp.html
). Feel free to contact our support team at email@example.com if you have additional questions.