I have a recipe that calls for "6 large baking potatoes." So the question becomes, what is considered the average size of a large baking potato? I've looked online, but sadly do not have time to continue my search and the results appear to be all over the place.
Tough question - there are a lot of non-scientific ways to measure small/medium/large potatoes and nobody seems to agree on exactly what weight each is. However, there are a few ways I identify large vs medium at home.
In my experience, a large potato is about the size you would get in a restaurant (I know, not very specific). That is, when I cook at home, the potatoes I purchase tend to be medium - round (not oval) and about the size of a tennis ball or baseball. When I'm at a restaurant their potatoes are noticeably larger, and generally more oval shaped it seems (closer to a sweet potato in shape and size), which makes them great for stuffing like for a loaded baked potato. Luckily most of the recipes I have at home that involve potatoes don't require precise measurement, so I just stick with my eyeball measurements and don't even bother weighing them.
I found a source online that says this:
And I think I can get behind that answer. I'd say your average computer mouse (like, one you would get with a new PC, not a fancy third-party mouse) is about the size of an average medium potato. Another way to eyeball: I'm a 30 year old male with hands that are neither very large or very small. The medium potatoes in this bag are about the size of a loose fist for me. So a large potato would be bigger than either of those things.
As for measurements, I have a bag of medium potatoes and it says they're 5.3 oz (150g) a piece. I weighed one and it was closer to 6.5oz (185g). Based on that I'd say a large potato would be anything over 8oz (225g), pushing as high as 12oz (340g).
I found a way to search online for this info: calories. By searching for "large potato calories" I was able to find pages with weights and measures:
So, in short, it seems like I would consider these good numbers:
Below is a picture of the medium potato from my bag, compared to some common household/kitchen items. The gift card is the size of a normal credit card, and the knife is a common steak knife from my utensil drawer. Apparently a medium potato is almost exactly the size of a 1 cup measure!
It probably depends where the recipe originated. My Northern Irish in-laws gave me a recipe for Irish stew that called for 6 medium potatoes, and when I made it everyone complained it wasn't potato-y enough! It turns out that their idea of a "medium potato" is WAY bigger than mine.