Hi,
1. The size is something we need to display (and will for sure

) on Source website , for now there is no way (except by downloading and looking into the sbs file for example) to know if this information is available on a material or not. It's something we started to add in April 2017 (just after the feature was added to SD).
2. The scale/size of a material is only an additional information, we however handle it in the MDL materials to scale it automatically if the "Physical Size" option is enabled, however it will not take into account your mesh UVs for example.
Also, as the amount of bricks you choose can have an impact on it, it's difficult to define an absolute scale for a material (because you can imagine to totally change it in some cases).