Standard in UK is 2 gallons (UK gallons, so 2.4 US gallons).
Older toilets in Australia can have an uncontrolled flush of 12 litres, but modern ones have variable flush. Normally there are two buttons on top, marked for full-flush and half-flush, but in fact neither empties the cistern. The half-flush uses 3.5 litres (less than one US gallon), while the full-flush uses 4.5 litres (roughly one UK gallon). If you have something truly horrible to get rid of, you
can press and
hold the full-flush button which empties the 11 litre cistern, but that is rather wasteful. One should get rid of skid-marks with a toilet-brush, not multiple flushes that waste water and rarely work anyway.
Many toilets use 5 gallons (in the U.S. many other countries have lower water pressure toilets, which is why in many countries you have to throw out your toilet paper instead of flushing it), but despite that, apparently I'm still gross. Thoughts?
The problem with getting rid of toilet paper is not typically one of water pressure; water closets are generally fed from a cistern mounted above the pan which is only filled by the mains, or other water supply, until the float-valve shuts it off, so the pressure feeding water into the bowl is only that generated by the difference in level between the cistern and the pan (so the old high-cistern, pull the chain systems had their advantages). The problem of getting rid of toilet paper (apart from gross over-use of the stuff), is the capacity of the pipework and sewage system to which the toilet is connected.
Many toilets in Asia still hark back to the days when toilet paper was not used, and users were expected to
wash their bottoms with water, so they don't handle TP well at all. You will often find a bucket next to the toilet into which you should put your paper. These toilets will usually be "squatters", however, so the TP bucket might be the least of your challenges...