yes
Daybeds are designed to accommodate standard twin-size mattresses. The standard dimension of twin-size mattresses is 39" wide by 75" in length, so look for twin-size fitted and flat sheets to fit your daybed. A daybed is fitted with a standard twin mattress. A daybed is designed to function as a piece of furniture in a den, guestroom or studio apartment and offers a place to sit that can be converted to a bed as needed. Standard twin sheets fit the daybed mattress. There are special dust ruffles for daybeds, depending on how high the mattress sits on on the frame. You can often find daybed linen sets at stores that have an extensive line of bedding accessories (Bed,Bath & Beyond; Linens and Things; The Company Store).
The size mattress that goes in a daybed is a twin size mattress. A daybed is piece of furniture that can be used as a seating area or a sleeping area.
"Daybed covers are available in twin, full, queen, and king sizes from most vendors. Use your mattress size to determine the size for your daybed cover."
the front part of the bed pulls out, and if you have two twin mattresses, it actually becomes a king-size bed. we have 2 thin twin foam mattresses stacked on top each other, and lie them side by side to make a king when it is pulled out.
38x75
Yes
Standard Twin Size: 38" x 75"
Yes, conventional daybeds are designed to use standard twin mattresses, so they can use any bedding meant for twin size beds. The twin mattress dimensions are 39" x 75", so you can check if that matches your daybed.
"The size of a daybed should be the same as that of a regular bed. If it isn't, you can buy a cover at your local household supplies shops. They should be able to help you find a daybed cover suitable to your liking."
because diffrent companies make diffrent size mattresses or diffrent size bunk beds
I believe a Hollywood bed is narrower than a standard twin size. I bought a "Hollywood High Riser Daybed" many years ago, and its mattresses are only 32" wide while a regular twin mattress is 39" wide. The length is about the same as a standard twin. When the two are placed together, they are about the width of a queen size bed.
Full size mattresses are about 75 inches long, or 6 feet, 3 inches. They are the same length as twin size mattresses.