Mine are 19 3/4 inside of open space.

Assuming by depth you mean the top-to-bottom measurement, it will depend if you use side-mounted or under-mounted drawer slides. Side-mounted will give you more height to each drawer, but you will have less width. Under-mounted is the opposite (less height, but more width). It will also depend on how wide the rails are between each drawer. As for the minimum height that is functional for the top drawer, that depends on what you want to store in it.

My inset drawers are 3 inches deep -- that is, 3 inches of usable space.
This is shallower than my drawers in the old kitchen
and it took a week or two to get used to the new shallow depth.
They work fine for silverware, utensils (even a potato masher), potholders, plastic wrap and foil rolls, etc.

Yes, it would have been great to have 3 1/2 inches of depth, or even 4 inches. But 3 inches is fine.

Depends on the cabinet maker- We make custom and when we do inset we do all of the top openings 1" taller (6") so that the drawer front does not look too short. Then we divide the rest of the height out to the two or three additional drawers. We use 1.5" frames so that goes to the math. I am not a big fan of 4 drawer bases unless you have a lot of small stuff to store. I prefer the 3- small top and two large bottom drawers.

Travis in NC

Thanks cribbs! That really helps! I will try drawing in the three unit with a six inch slab for the top drawer. What do you do in a baking area where the counter is dropped a few inches? How do you make those cabinets line up with the rest of the kitchen?

Plain & Fancy has a 5" drawer OPENING while Shiloh has a 6" drawer opening.

I prefer slab drawers on all top drawers. For a more formal look, you can do a drawer head totally different. P&F offers a drawer that is recessed with a small molding applied to the drawer....see picture below.

I'm not so sure I like the 6" high openings as well. They are better if you want a matching doorstyle drawer front. Usually top drawers hold smaller things like silverware, utensils, saran wrap.... Really no need to have them an inch deeper.