600 sq foot apt here in Astoria. We buy probably 70%+ of our groceries at Costco. It can be done. Focus on the most dense stuff with high value / weight ratios (think: meat) or the stuff that you will always, always need that doesn't go bad/is freezable. For us, we buy:
-Flour
-Rice
-Eggs
-Butter
-Chicken
-Beef
-Sugar
-Coffee
-Bacon
-Sausage
-Frozen fruit
-Frozen veg
-Toilet paper/paper towels
-Olive oil
-Vinegar
-Vegetable oil
-Canned vegetables
-Pasta sauces
-Cheese
-Pasta
-Honey
-Nuts
-Movie Tickets
-Toothpaste/Shampoo/Conditioner/shaving cream in the bathroom
We figured out that we save something on the order of about 50% by shopping at costco for these categories, or several thousand bucks a year. Also, having a fully stocked fridge/freezer/pantry has drastically reduced our urge to eat at restaurants.
The easiest thing to start with things that definitely will not go bad: freezable stuff or non-perishables. Then phase in stuff that is perishable but you will go through quickly enough for it not to spoil. Buy all the other stuff (where you need small amounts, or fresh ingredients) at Stop & Shop or whatever.
Doing our system has seriously saved us probably $2,000 or so over NYC grocery prices (we have stopped keeping track, but $2,000 would be the minimum we save). As far as storage, get lots of nice re-usable containers/bottles for stuff you use often. Refill them from your huge ass costco supplies periodically. Keep your huge ass costco stuff in the least accessible parts of your apartment and only use them when replenishing your daily use containers.
Trying to grind coffee from a bag this big will make you feel like an asshole, and you will hate costco every time you use their stuff.
Grinding coffee from a jar like this makes you feel civilized. Use re-fillable jars/bottles like this for all the basics: sugar, flour, rice, coffee, olive oil, vinegar, etc.
Meat is by far (for us in NYC) the biggest savings. Go completely apeshit in the meat department at Costco. Invite your friends, have a party, take selfies and put them on instagram. Clear out your freezer and tetris the hell out of it with all the stuff you get at Costco. Whenever you come home from Costco, just divide up the huge amounts of costco crap into ziplocs with enough for a recipe, wrap them nicely, and freeze them. We wrap a 6 pack of steaks individually, divide ground beef into 2 pound "pucks", etc. Only takes about 10 minutes when we get home from Costco, and then just take a single serving of whatever you want to cook and put it in the fridge one day prior to cooking.
Hope this helps.
EDIT: We are a 2 person couple as well, just the two of us. Seriously. You will save crazy fucking Oprah money in NYC. Get on board with this!