We are sorry we were not able to help you in time. In fact our entire support service has been missing for the last 30 days due to company issues. For a very short period we’ve left without any staff to handle requests and it took us some time to resolve this problem. We’ve now managed to restore our service and support will be provided with the normal rate.
About your problem: product size can be achieved with product options. This is a default OpenCart feature and can be accessed in Admin > Catalog > Options. Here is a video tutorial how to use product options: