I think, in English at least, dinner is the main meal of the day, whereas supper is always the evening meal. For example someone may invite you for Sunday dinner, in which case the noon meal being the main meal of the day would be referred to as dinner. However in most other cases I would always think of dinner as being the evening meal, since it is usually the main meal of the day.
In Swedish, middag is the main meal of the day, and then there's kvällsmat in the evening if you eat a lighter meal at that point. (as I said this is the standard usage, so it's a simplified picture, but this is how most people use the words).
Most people eat middag after work, maybe an hour or two after coming home from work.
Take a look at the wiktionary article and open up the Inflection Table. I use wiktionary for most new words I come across - its a great resource!
It will tell you that stor & stort are for describing singular en & ett words, and stora is for describing plural and definite nouns. In this sentence, lunchen is definite.
Ett stort bord / A large table
En stor lunch / A big lunch
Den stora lunchen / The big lunch
De stora luncerna / The big lunches
(If I got something wrong please point it out).
It's called double definiteness, but actually it's more like triple. When there's an adjective before a definite noun, the standard is to express definiteness in all three places: the front article, den, the form of the adjective, stora, and the end article lunchen.
There are some special cases where it doesn't work like this, you'll learn about those later on in the course.
It's "stora" no matter if it's an en or ett word. "Store" is an optional definite that you can use if you're definitely talking about a male. Not in the French sense where all nouns are masculine or feminine, but only about people (or animals, possibly) that are male in the literal sense.
Here's the difference that I have found between Duo on a computer and Duo on a phone: If I log into DuoLingo using a computer, I get both the exercises and the grammar notes. If I use a phone app (and I think this true of tablets, too), I get just the exercises. I CAN get to the grammar and full Duo content using my phone by clicking the "View Discussion" link in any discussion thread email. I click on the little bars at top of the screen and then log into DuoLingo. Et voila, it's all there. Hope this helps.