The scale is,
IIRC, from the ground to their eyes. So, 28mm is roughly 30mm to the head. Add a bit more for
GW models, though. They are 'Heroic scale', or some-such.
Have a look through the gallery. Lots of people do comparisons.
I have done a few, for buildings in this case:
http://www.dakkadakka.com/gallery/images-38888-22076_Stuff.html
Talking of cases, carry-case companies make custom cases for certain models, and you can see the size of the models by looking at them:
https://www.krmulticase.com/product.aspx?id=SM1+SM21&t=tray
https://www.krmulticase.com/product.aspx?id=SM5+SM7&t=tray
There are also video clips showing people painting the models. You can work out the sizes as they handle them in the vids.