According to Bed Bath & Beyond, there are a few things you have to know first before you buy items for for your bed. Begin with determining the type of mattress you would like to have and then you can move on from there.
Sheets. The measurements below are for fitted sheets and will help ensure a proper fit. Do not be concerned with the top or flat sheet dimension, as it will be tucked under the mattress anyway and is not meant to be sized to the exact specifications of your mattress.
Mattress Depths. Pillow Top and Extra Deep mattresses have become very common. Before selecting your sheets and mattress pads, it is important to know the depth of your mattress. Here are some dimensions to use as a guide:
Comforters. Don’t make yourself crazy looking for California King Comforters or Twin X-Long – they don’t make them. Just get a King or Twin size accordingly. In addition, when looking for Full or Queen sizes, some manufacturers make comforters in both Full and Queen sizes, while others will offer a Full/Queen size, which fits both mattresses.
Daybeds. Since all daybeds are outfitted with a twin mattress, selecting the sheets and mattress pads for them is a no-brainer. Choosing what to put on top of the daybed can be a bit more challenging, as sizes and styles can range. There are daybed covers, which typically have three sides (two sides and the front), and daybed spreads, which typically have four sides, and are usually quilted. The “drop” of a daybed cover, or how far down to the floor the front of the cover reaches, will determine if you need a dust ruffle, also called a bed skirt. A standard twin 14” dust ruffle is all a daybed requires, though some dust ruffles feature split corners to make navigating around bed feet a bit easier.
More great ideas here.