Size of quilt ?
 

I am making a quilt for a benefit. The blocks are 12" finished. The backing I want to use is 56" wide. So I have to make the rows 4 blocks wide for 48" wide. Will this be sufficient for a twin size quilt? Otherwise I could add a 2" border also. I don't want to piece the backing. Thanks for your advice.

a twin mattress is 39" wide. I would think most twin quilts are at least 60" wide.

I would go with what you have. If it is for benefit, are they going to turn down whatever you make? I think adding a border but keeping it within your backing width would work fine. I have done this myself for baby quilts. Much easier than piecing the back.

I've just been looking at a pattern that I have that can be made in three sizes. According to this pattern; crib 44 x 56; lap 55 x 70; twin 66 x 84. Of course it is your quilt and you can make it any size you want! :)

I make them at least 70 to 75x90 for twin beds.

I have never heard of 56" width of fabric before. Make it the size you want to but it won't be a twin but more of a lap size.

Standard twin quilt is 68 " to 72" X 88" to 90." No, I don't think it will be big enough for a twin size quilt, but it may cover a twin size bed.

It's good of you to be making the quilt for the benefit!