“Maybe different cafes do different shot sizes.”
Yes, different cafes us different doses, some cafes dose different coffees at differing doses.
“Shot size” relates to the liquid amount, or brew ratio.
Ratio, when using the appropriate dose for your basket, is what you want to focus on.
16:48 is essentially the same as 18:54 assuming both are extracted similarly, bigger dose = bigger drink.
Lighter,brighter coffees might benefit from longer ratios compared to those with lower acidity.
But is there a specific reason why you ask?