If you use the Designer, when you create a NEW VARIANT you will see the available sizes, however, you could also use the Designer Scripts to scale your layouts on every screen size, which in my opinion, is a better way.
I think I understand what you mean, but this thing is usually done with the 'designer'. You can choose a variant matching the dimensions of the 10'' tab or any other. If you have it, you can even connect it with the designer and choose 'layout' -> 'match connected device'.
If you use the Designer, when you create a NEW VARIANT you will see the available sizes, however, you could also use the Designer Scripts to scale your layouts on every screen size, which in my opinion, is a better way.
A new version 2.00 is available in the first post here.
Updated for tablets.
Works only in portrait on phones and both orientations on tablets.
Needs Basic4Android version 2.20 uses AutoScale.