How much does a "large" potato weigh?
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:


  According to Shape magazine, eatbetteramerica.com and prevention, a medium potato is the size of a computer mouse, so somewhat bigger than that. 


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:


3 to 4 1/4 inches in diameter
300 grams (about 10.5oz)


So, in short, it seems like I would consider these good numbers:


Medium potato: 2 to 3" in diameter, 5-8oz (140-225g)
Large potato: 3 to 4.25" in diameter, 8-12oz (225-340g)


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!