Hi Jelmer, I am not sure what you are doing wrong but my guess would be that you have a mismatch between the platform you are generating your Soundbanks for and the platform that is selected for your Wwise project. In the upper left corner of the Authoring, there is a platform selector, where you can choose between Mac and Windows (in the case of Lesson 7 project). If for instance you have the "Mac" platform selected but that in your Soundbank Manager you check the "Windows" platform box and then generate banks, you will never see Data Size for your generated soundbank because you are currently inspecting stats for the "Mac" platform. In order to be able to see Data Size for the correct platform, make sure the platform selector drop down is matching the platform you are generating banks for.
I would also like to mention that it would be better if you used 2017.2.x to follow the certification course as the course and questions are not compatible with 2018.1 yet.
Hope this helps !