Final answer:
The default size of a burger at a restaurant's register depends on the restaurant.
Step-by-step explanation:
The default size of a burger at a restaurant's register would depend on the restaurant. Some restaurants may default to a single patty burger, while others may default to a double or even triple patty burger. It is important to note that this can vary from one restaurant to another and it would be best to check with the specific restaurant or consult their menu to know the default size of their burgers.