"Show Points" and then select the 4 points of the top surface, then in the upper right corner where the entity summary is, click on the size label and then type the new size in there.

Usually "Solids" are not going to edit in this way. You need to create it the Shape you want. Danny did a great little tute on creating a draft angle using "Chamfer with 2 distances".

"tan(20)*Distance"

I'll see if I can find the link and post it.