in general what is the standard size for an interior household door?
Having just bought five interiors doors recently. I can tell you that it depends. My Bathroom doors are 24 by 80. One of the bedrooms is 30 by 80, one is 31 by 80, and one is 32 by 80. If you are going to just buy the door itself and not the prehung with a jamb, you should measure what you are replacing.
Are you talking a single family house or a condo? Big difference. Condos and any multi family housing will fall under the Fair Housing Act (FHA) and thus will likely have a minimum 2’10" standard door for an interior door (FHA requires a 2’8" min opening).
Single family it can be anything since they are not required to meet the FHA. Typical single family is 2’6" wide for most doors, but some bath doors can get down to 2’4 or 2’0".
Entry doors are typically 3’ wide.
Vertically standard door is 6’8" but many doors are now 7’-0" tall.
hope that helps! 
2’6" & 3’ x6’8" used to be the standards for entrances and passageways--------------anything else is a matter of what the pre-hung factory turns out that day,either by special order -------order-or by dis-order![aka OOPS]
EZ
used to be being the operative word
when the FHA passed and as an Architect I had to start putting in 2’-10" wide doors in my multi family projects it was a huge deal–now it is a standard size. I rarely use a 2’6" door these days. I also rarely use a 6’8" high door. My current project has 8’ high doors. Part of this is that ceiling heights are greatly increasing. Years ago an 8’ ceiling was a standard. Now this is considered inferior. My current projects tend to be 10’ floor to floor which leaves a ceiling of about 9’4" or so. A 6’8" door would look odd in that situation.
Other than saying “there isn’t” we haven’t done a good job of answering the OP. While taller ceilings dictate taller doors, and 6/8 is something of a standard, developments of northeast Phila have 6/6 and 6/4 doors in sufficient numbers that Home Despot stocks them.
That’s interesting. If you look at ceiling heights over, say, the past 100 years, they’ve gone up and down a lot. I know that in the ‘60s, at least her in Washington new construction standard cieling height was 7’ 6". I’m glad to see that changing.
(psst, “used to be” is three words") 
Sorry, carry on…
we are just lucky that the ceiling heights advocated by Frank Lloyd Wright didn’t become the standard. If you have ever been in his homes, most people have to duck If I recall correctly he was a shorter man and he used his height as the measurement of what a good ceiling height should be.
(psst…it is spelled ceiling no cieling)
I knew that. I just wanted to see who was paying attention. 
I have been in one of Frank Lloyd Wright’s homes, the Barnsdall in LA. I was amazed. Even though it has such an open design, it felt cramped and dark. I didn’t actually notice the ceiling, but that explains it.