When a recipe calls for a “small onion” or a “medium tomato” or something tlike that, how do you determine if the ingredient you have is the right size? What makes a vegetable or fruit “small”, “medium” and “large”?
Seriously, unless you’re baking cakes or pastry or something the amounts in recipes are pretty much advisory. When it comes to vegetables, it’s all about how much you like the item in question. I like onions, so if a recipe calls for one small onion I’ll probably throw in a huge one or maybe two or three little ones. I don’t like tomatoes raw so any recipe that calls for them will get the eentsiest Roma possible so the taste is right, but no more. For most veggies something the size of a woman’s fist is close enough to “medium” for government work. If it’s bigger than your head you probably shouldn’t eat it… 
Thank you!
Small and large are typically judged by the range of available sizes - for example, in the case of onions (and excluding shallots and pickling onions, etc), they’re typically available in a range of sizes from about 4cm (golf ball) diameter, up to about 15cm or so, with the ‘normal’ size being something in the middle - say about the size of a tennis ball. So a small onion is something noticeably smaller than a tennis ball, or that holds true where I live anyway.
But SmartAleq is right - unless the exact amount is specified by weight or volume, it isn’t usually critical, so you can use a bigger one if you like that ingredient, or a smaller one if you like the others.
You have in front of you three onions. One freaking huge one, on little tiny one, and one that is in the middle.
One is too big for the recipe, one is too small for the recipe, and one is just right.
Not in one sitting anyway… 
Tennis ball size sounds about right to me for a medium onion or tomato; I usually figure medium means “enough to know it’s there, but not so much as to be overpowering.”
I’ll admit it, I hate recipes that that request veggies of various sizes. I especially hate it when they request large potatos, onions, etc. Or multiple large or medium potatos. Especially for potato salad, I wish my recipe would give me a volume amount (like 3 cups) as well as an estimated number of approximate size potatos.
On the other hand, I agree with others that in general the recipe will work equally well with a smaller or larger potato, onion, etc.
Cooking is more an art than a science. Painters don’t mix paints based on grams or liters. They mix the colors until it looks right. If it looks like there is going to be too much onion, don’t add all of it.
Baking on the other hand is like chemistry. If you don’t have your measurements just right, it is going to either blow up or fall flat.
Medium tomato = exactly 3.211 oz.
Yep. Some of my best meals came out of desperation, taking what I had on hand and trying to make something delectable out of that.
Stand in the produce aisle, and ask, “Can any of you help me contact my late grandpa?” Any onion that answers “yes” or “perhaps” is a medium onion. 
I’ve got one that is 3.212 ozs. can I still use it?
Rick,
Yes, but after you cut the tomato up and before you add it to the pot(bowl, whatever), remove exactly one tomato seed and throw it in the trash.
I once struck a happy medium, got six months for assault.
Okay, AskNott wins the thread.
So long as you have a microtome, sure. Just slice off about 0.03% before use.
If following a European recipe, a 91.0g veg is required.
If you think that’s vague. I have an old recipe for a cake that says it should be baked at “medium” temperature. 
We figured out it should be baked at 325
I have a ton of old recipes like that.
Bake in a medium oven until done.