Pillowcases and pillow shams are usually about 3 to 5 inches (8-13 cm) larger than the pillow itself, so the pillowcase size just needs to be close. Here’s a reference to keep things straight: 

Similar mattress sizes may have different names in different countries. Mattresses of the same name may vary by a few inches in different countries; sheets will usually fit if the mattress is approximately the same size – within a few inches or centimeters. Mattresses from different manufacturers will have different depths, so be aware of these when selecting your fitted sheets. Use your mattress size to determine which size memory foam mattress topper or feather bed you need. 

Your fitted sheet needs to be as close to your mattress size as possible to ensure that it will fit onto your mattress snugly. The pocket depth of fitted sheets may range from 10 inches (25 cm) to 18 inches (46 cm). Be sure to measure your mattress height before selecting a fitted sheet. 

Flat sheets are usually sold in sets along with fitted sheets. It’s most important that your flat sheet is wide enough. If it’s a little long, you can fold it under the mattress. If you use a duvet with a washable cover, you don’t need a flat sheet. 

In British English, a duvet is known as a quilt, while in American English, duvets are down comforters which require a cover. Duvet covers are also known as quilt covers. The covers are meant to be used without flat sheets or other blankets. Duvet covers do not need to be a precise fit but should be as close as possible. 

Crib mattresses are 28 inches by 52 inches. Crib bedding needs to be an exact fit to the mattress for the safety of the child. Toddler mattresses are the same size as crib mattresses; the frame is just set lower to the ground, so the child can get in and out of bed independently. You can continue to use the same size sheets but will probably want to add more blankets to the bed.