I made mine big enough to fit the largest object I wanted to store in them and no bigger.

Space in a van is too important to worry about how things are "supposed" to be. Even a few inches wasted is noticeable.

 

cardboard. Make a mockup and feel it. Only way you'll know what works for you,

 

After rigging up some frames to empirically set the skull clearance, we set our upper cabinets front height and depth to 13".
Here's the exact quote from the blog entry:
"There are two cabinets over the counter. they are 13" deep at their base, and 13"high at the front. Since the wall and ceiling are sloped, things narrow down toward the rear...to approximately 12" deep and 11 3/8" high. "
Here's the link to the entry.
Building the kitchen upper cabinets

All our overhead cabinets are built to the same frame dimensions. The widths vary depending on what we planned to store.
Cabinet frames are made of maple.

 

Since the wall and ceiling are sloped, things narrow down toward the rear...to approximately 12" deep and 11 3/8" high. "
[/QUOTE]
Do you have any tips for how you arrived at the "correct" angles so that the front/bottom of the cabinet is perpendicular/parallel to the floor?

 

Do you have any tips for how you arrived at the "correct" angles so that the front/bottom of the cabinet is perpendicular/parallel to the floor?

Yeah, it was empirically.
I'm using box joints in the cabinet construction, which have the convenient feature of serving as hinges. I knew the distance I wanted the front bottom corner to be from the wall & ceiling(13"x13")
Once I cut and box jointed the front and bottom legs, I set the bottom/front angle to 90deg. Then, temporarily held the prototype front/bottom segment of frame up against the wall & ceiling (ensuring the bottom leg was parallel to the floor and the front leg perpendicular), I could measure the outside length of the upper and rear legs of the frame.
Was handy to have spousal unit (and aesthetics committee chairperson) holding the frame (and a level) in place.

Then I verified by measuring twice, cutting and jointing the upper and rear legs, measured and cut/jointed again.
Actually, the "hingeyness" of the box joint allows for a good range of setting the rear angles and lengths. Once that was done for one side frame, I cut & jointed the legs for all the other cabinet frames.

 

To determine the best location for height and depth I installed a rope where the bottom front of the cabinet would be located. Moved the rope in/out and up/down until I found the best location. Then built the 80/20 cabinet face frames. Hung the cabinet face frames from the roof ribs. Installed strings vertically from center of roof rib to center of the floor. Measured from string to the top & bottom of the cabinet face to get it square. Then made the cabinet plywood floor between the cabinet face and the wall. Next I made the plywood cabinet ends. First made the cabinet ends using cardboard until I got the size needed and used the cardboard as the pattern to cut the cabinet end panels.

 

We built the frame and then changed it. Until it felt right for that location (first one - above the fridge). From there, we'll be doing different sizes for depth and height depending on location: same height, shallower depth along sides of bed; higher (drop down below top of slider) an slightly deeper above the galley.

No front cover on the cabinet in the photo - heading out for a week and didn't finish it. Empty - just checking to see how well the iron skillets would sit in there; then the temporary setup loaded up for a trip. Hinged (up) door goes on next.

 

I haven't started mine but will be using this design from Tim and Shannon on youtube .Open cabinets using baskets but doors could easily be added afterwards if you were unhappy with them open. It also addresses your measurement questions