So occupancy is, firstly, based off classification. An IKEA would presumably be M under the UCC (Uniform Construction Code). Capacity is determined using the amount of exits and distance to those exits. You can have a million square foot building, but if there’s only one door, you’re going to be limited to 50 peoples. That’ll obviously never happen, but just an example.

Mercantile can range from as few as 30sf per person to as many as 300 sf per person. Assuming this Ikea is on ground level, has a bunch of doors, I think you can safely use 60sf per person (10x6 area), which would allow for about a bajillion people in a 300k square foot IKEA.

That load is kind of in the middle. There are uses that require greater separation distances, like hospitals and other institutional uses, and then some that require less. I threw out 100sf per person, because 10x10 is honestly a bunch of room for your average person. I think you could reasonably do some cartwheels in a 10x10, and to me the cartwheel is a great demonstration of personal space. If you can cartwheel, you’re good to go. What more do you want?