Shrimp are sold based on their size. The sizes are designated by determining how many shrimp you should expect to find in a pound. For example: 21/25 = you will have 21 to 25 shrimp in a pound.
When you have been cooking shrimp for a while you start to realize what size shrimp are most appropriate for various dishes.
I enjoy using 21/25's most often for my family dishes because I can cut them into thirds and sauté with veggies for a perfect and pretty consistent shrimp meal every time. My kids are still young and if I used whole shrimp, I would be expected to help cut them into bites when I plate them. After cooking a meal, I don’t want to spend a bunch of time cutting the food up for my kids. I prefer to do it all at once.
When I am entertaining or making a dish for an ALL ADULT meal I use 26/30 or 31/35 so I can have plenty of whole shrimp (makes a better presentation) in whatever dish I am preparing.
The smaller shrimp such as the 51/55 are great for various appetizers , to cook into nice red gravy to top spaghetti squash, on top salads or fry into popcorn shrimp. I do not however enjoy peeling these smaller shrimp which is why I'd rather cut the bigger shrimp into 3rds when “presentation” isn’t as important as the nourishment and flavor.
16/20's are the perfect date night shrimp or to pair 4-5 of them with a small bit of steak for a perfect surf-n-turf for an ALL ADULT meal. When they get much bigger I like to boil them and puree into fantastic shrimp bisque. Hope this helps.