To actually answer your question...
- the normal tradable/usable gold bullion bars are "400 ounce" bars. Aka LBMA, mocattas, LGD bar, etc. (Note that as a curiosity, they are not 400 ounces, they each vary. If you're ever talking to some fool who thinks they are a gold bug and has never actually owned any bars, you can trick them with that question :) )
(If you wish to know why they are called LGD etc, read a boring wiki article.)
- the normal tradable/usable small bullion bars are one kilo bars, as mentioned in the question
Just as you suspect, there is a smaller size:
- the only tradable/usable smaller bullion is indeed one ounce rounds. You can buy and sell these instantly for very near spot anywhere in the world - at the blink of an eye.
The generally famous bullion rounds are these three: the canadian maples, the lovely aussie kangaroos (they also had a fantastic chinese year series), and the britannia.
Note that there are other 1 ounce gold coins which have one ounce of gold BUT they are made of an alloy (ie, the overall weight is a bit more than an ounce). These are cool and fairly easily sellable, but stick only to pure-bullion rounds (the three mentioned) for the most easily sellable situation. Got it? Good!
That's it. There are also novelty smaller sizes, but forget it.
Footnote: a 10oz size bar is somewhat popular in iconoclastic Switzerland. And indeed in enigmatic China there are certain oddball sizes that are slightly popular. But the above three are the way.
When the zombie apocalypse comes...
Let's add a little intelligence and finesse here. Sure, if there's literally like an asteroid strike it is meaningless.
What the OP means by "when the world ends" is a crisis typified by a hyperinflation - for example the German hyperinflation in the after-war years.
These happen all the time - every century or so somewhere or other.
In those times you'll be extremely glad you have lots of gold, whether ounces, kilobars or (if you're rich!) normal bars.
A trading note ...
Be aware that a couple years ago everyone with a clue was saying, buy gold, be a goldbug, it's gonna skyrocket
Take care not to jump on a fad which has already had a big run or run it's course.
And don't forget chain ...
There's already an answer here which (shock) has some facts on chain. Gold chain is a really great thing if you're a "dollar collapse is coming" type. If it's 100% you can buy and sell it very close to spot. You can also sometimes/often, uh, well, smuggle not inconsiderable value through national borders / war zones / etc. via chain.
Just wander in to Chow Tai Fook in Hong Kong, the spreads are incredible/nonexistent for bullion (ie pure) chain and knickknacks.