I live in a 225 sf apartment which has (3) circuit breakers on the outside box on the side of the unit (it's more like a bungalow actually). (2) of the breakers are 20A -- one is only for the bathroom and one is only for two ceiling lights and an outside door light. The 15A breaker is for the entire place besides what I have described, which includes the entire kitchen and (3) wall sockets in the living area. My question is: is this legal? I have a full size (21 cf) refrigerator, a microwave, a coffeemaker, a television, a computer (w/ monitor), two lamps and an air conditioner. I can run the a/c and use the computer while the fridge is running. Anything else (make coffee, heat food in microwave, watch a movie, etc) and the circuit breaks (duh... I know it's pulling a lot of power). I just want to know if this is just was a really bad DIY project (my landlord did it) or it's fairly common. I couldn't figure out how to post a picture of the breaker box but it looks ancient and doesn't even have a door covering the breakers--so everything is rusted.

If it has never been inspected for insurance or sale, it will depend on where you live. I owned a 433 sq' condo in Cambridge MA that had one ancient 20A fuse in the basement, as did every unit in the 63 unit building, and the management company had insurance on the building. I had a fridge, microwave, toaster, TV, VCR, computer, and even 6.5A air conditioner, and was careful, and never blew the fuse. If you want to call the building inspector, feel free, but plan on not living there past the end of your lease.

I wouldn't say it's common or smart, nor would it pass if the owner had pulled a permit, and I would be concerned about the rust if there is water dripping on it.

I just want to know if this is just was a really bad DIY project (my landlord did it) or it's fairly common. I'm not sure how we would know if the job was a DIY.
If the equipment is that old as you describe then it is entirely likely that it is original. I have seen homes with a whole second floor on one 15A fuse.
You have to admit, 225sq/ft is tiny by most standards.

I agree, I had something like that in NY. So I agree it may have been like that when it was built. Which of course makes it legal, but would have to be changed if they did any upgrades to system

May have been code when built but we will likely not be able to determine the build date. Are you having problems? I can't picture a frig and an AC unit working on one 15 amp circuit let alone your other appliances. If the owner di it he likely knows little about calculating loads or pulling permits.