That depends on how far you want the quilt to hang down the sides, if you want to cover a pillow, if the bed has a foot board, and even how thick the mattress is. Check out the website below for a quilt dimensions calculator. It's the easiest way to figure exactly how big to make the quilt. It lets you choose the standard mattress size, then adjust the length, width, height, mattress thickness, box spring thickness and even pillow thickness. You choose how far down the sides of the bed you want it to go, whether you want it to cover a pillow, and if the bed has a foot board, and it tells you the exact dimensions you need.
For example, a standard twin mattress is 39" x 75". If your quilt will cover the pillow, will hang down at the foot of the bed (no foot board), and is "blanket" size, which means it hangs to 1" below the box springs, your finished quilt should be 75" x 105".