About a few decades ago, the futon sizes were based on the unit of shaku (one shaku=36-38cm), which was originally used for the kimono sizes. Partly influenced by the Western culture, the futon sizes became later modified.
Here are the standard sizes of a wafuton.
Ｓ Single 150×200
L Long Single 150×210
SW Small Double 170×210
Ｗ Double 190×210
Ｂ Baby 100×130
Ｓ Single 100×200
Ｌ Long Single 100×210
SW Small Double 125×210
W Double 145×210
Ｂ Baby 85×130
Sizes of Futon Covers
When you buy a futon cover, be sure to measure the futon size that you are using.
Since the kake-buton (comforter) is usually thinner, you can use a futon cover which is similar in size to the kake-buton.
Since the shiki-buton (bottom mattress) has the thickness, you can use a little bigger futon cover with its dimensions 5 centimeters longer and wider.
As for a pillowcase, the size may vary depending on each item, so it might be good to buy a set of a pillow and a pillowcase.