The measurement for a twin size bed is 39×70 inches.  Bear in mind that is the ‘bed” size.  You’ll want your twin size memory quilt to hang over the sides and the end of the bed.  The standard is 12 inches but it can hang over as much as you want.  Assuming, you are making a twin size memory quilt, the finished size of your memory quilt would be 63×82 (39 +24 (12 inches for each side of the bed)  x 70+12 (the 12 inches hanging over at the foot of your twin bedbed).