How would a novice cook know
 

How much to add of an ingredient if the recipe says

A pinch
to taste
A glug
A handful
what is considered small, medium, or large for an onion or potato ( or whatever)
Difference between rounded and heaping measurements

So true. Some skills are caught, as much as taught.

I think it is mostly common sense. A pinch is what is between two fingers. To taste is what you like it to taste like.
A glug is one big dollop of liquid coming out of the carton (a mouthful). A handful is whatever the size of a normal adult hand. You can tell if an onion or potato is small, go from there. A heaping teaspoon is ingredient piled as high as it can go without spilling over. A rounded teaspoon is ingredient rounded off to fit the spoon. That is how I would measure all of these.

For recipes and carb/calorie and all that sort of thing, if it is round-ish (like an onion or an apple) "one" is about tennis ball size. For fruit that is roughly the size of the bags of lunch fruit, maybe a little larger, but I think an "average" orange or apple from the bin is more like 10-12 oz instead of 6-8.

I know what you are saying the recipe says 4 potatoes and 3 carrots you just go by what you would cook for your family

In my dotage the only things I measure are for baking some things. I can do biscuits and pancakes with eyeballing to see if it looks like a batter or dough should. The rest I have been pretty good at just my own experience and taste. I would say less experienced folks will use "the google" until they get an idea of things

Bear, I had to smile. I have one of my grandmother's hand written recipe books. How much is in a tea cup? Or 'use the silver teaspoon' measure. And 'less than usual' in a bread recipe can be quite perplexing. I remember her cooking--it was wholesome and tasty, but a culinary genius, she was not. She did make the most amazing chocolate and vanilla ice cream jelly rolls, and Long Johns! My sisters and I have tried but have never been able to replicate those two favourites.

Oh a tea cup is 6 oz. The tea cup always sits on a saucer.. I remember this from old time Home Ec class.

even novice cook books don't tell you to not boil peas. Unless you want mushy peas on your french fries.
Simple things like this would prevent a lot of sad first meals.

That's how my mom cooked and she couldn't figure out why I could never learn to cook from her. When I was on my own, I'd get recipes with the actual amounts. 1/4 tsp., 1 T, 1/2 c. I had no problem cooking when someone told me exactly how much to use. When you're starting out, a "bit" can seem like anything from 1/4 tsp to 1/4 c!