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."
38x75
Yes
Standard Twin Size: 38" x 75"
T30, 2 of them
10 mm
yes
"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."
8mm
A futon would be better as it can double as a sofa.
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).