amphibious, I'll guess you mean 2 appliances, both under the counter: "... about 6.5 cubic feet of fridge space and 2.5 cubic feet of freezer space, which means two units. What I had envisioned before I started doing research is one sole refrigeration unit and one fridge/freezer. Little did I know that the fridge/freezer units would be so hard to come by..." The more motors, different cooling circuits and separator walls you try to cram into a small space, the less space you have left over to call fridge or freezer. I got a 36" unit, for that reason. More real space left over, after the insulation and mechanical stuff. Your perceived space needs are actually in a range, quite elastic. Go for a fridge unit and a freezer unit.

I have the Electrolux Icon all fridge (2 of those) and 1 all-freezer. (Electrolux is the parent company of Frigidaire, who also makes the Kenmore). They aren't big, but I've had full depth fridge/freezers and these give me more room with more accessible reach and better visibility. Cu ftage measurements don't tell you much unless its 'usable cubic feet,' and I think counter-depth, plus good organization (which might vary from Frigidaire to Kenmore to Electrolux because of different features) helps the usability. You'll want to compare the different names, in person if possible, to see which of those has the shelf options, bins, etc. that best suit your mother-in-law's needs. By the way, we have 2 of the fridges, because we have 8 kids. It's rare, even in our house that both fridges are completely full at the same time, so 1 would probably serve most more modest sized households pretty well. The freezer, if she's used to a full size freezer, might be smallish.

I have only posted a couple of times. I'm certainly not an expert but I have spent countless hours/weeks on this forum and have gained so much knowledge. Thank you all. We are nearing the end of our renovation. My countertops are going in right now. I hope I like them. I had originally planned to do the SZ 736 and a set of under counter frig/freezer drawers. I then found a picture of a refrigerator armoire I wanted to copy. I didn't have room for an additional 36" of refrigerator or pantry space to achieve the armoire look. I ended up doing two side by side 27" SZ 700TCI Refrigerator/Freezer units. It made a nice sized refrigerator armoire and I got more refrigerator and freezer cubic feet than I would have had with the SZ 736 and two under counter drawers. The two 27" units gave me 20.4 cubic feet of refrigerator space and 10.2 cu ft of freezer space (4 freezer drawers). Two of the freezer drawers have ice makers that are removable. On an everyday basis, I'll just have one ice maker in use. If we have a party, I can add the second ice maker. You could also do the 27" all freezer unit combined with a 27" all refrigerator unit. The other nice thing about the 700TCI unit is that the motor is on the bottom. I was able to put 3 pretty glass cabinets on top of the two units. We added some crown molding and it is like a piece of furniture. This doesn't answer your question about how much refrigerator space you need, but it's another option.

36inch counter depth (builtin or not - you didn't specify) is not a lot of space for a family of 5. If you get a combined fridge/freezer, you lose extra space for the partition and the extra electronics. Space-wise, it would be more efficient to put in an all-refrigerator in the kitchen and an all-freezer in the garage, then a combined fridge-freezer in each location. It would also allow you to store wide items in both the fridge and freezer. That might also be less expensive because there are many inexpensive all-freezers and a freezer in the garage does not need to look fancy. Whether this works depends on how often you need to get into your freezer. The other consideration is that you probably don't want to have to go into the garage for ice. If you have a separate ice maker or a freezer drawer with an ice maker, that would solve the problem.. You could put two freezer drawers or a combo unit (one fridge drawer and one freezer drawer) under the counter. If you have a separate ice maker, you could even put in a beverage fridge under the counter to free up some space in your main fridge.