Espresso is weight based so yes it is only one size. You have to extract the espresso from the ground espresso bean using something called a Portafilter which can only fit so much ground espresso. Doing this requires a fixed amount of pressure and water to "pull the shot" of espresso. Because all of their drinks are based on this fixed espresso volume, yes there is only one size.
If you want a larger volume then you can get an Americano which is the espresso shot diluted with water which is what I usually get as I like to sip on it a big longer and I guess this would be equivalent to a medium coffee at other places.
If you get a "Pourover" which is fresh coffee bean grinds with hot water poured over them to allow them to "bloom" then further extract the flavour with more hot water. This might allow you to get more coffee equivalent to 2 cups or a large elsewhere? Depends on how much coffee grind and water they use. Often times there are optimal ratio of coffee grind:water that you want to achieve to get the best flavour.
https://bluebottlecoffee.com/preparation-guides/pour-over
Espresso is a slightly different concept than many Canadians are accustomed to with the coffee that we brew at home or places like Tim Hortons. However, because it uses higher end beans and extracts the coffee with higher pressure, it results in a more flavourful experience in my opinion.