Ok I understand what you mean,
Right now, the solution I can think of is using a grid on your section and using the calc option on your grid row to subtract the toolbar height from the 100vh (I understand that this varies between different devices).
There is actually a way in Velo(wix-code), that you can get the height of the document , but at this moment you can not take this information and apply it directly to an element or container.
I will raise these feature requests for future consideration.
Thanks for bringing this up,
Ido.