If my calculations are correct, a 3 x 4 block quilt where the blocks are 17.5 inches will result in a 52 x 70 quilt which is plenty large for a snuggle quilt--especially for a small 12 year old girl. It will be twin size if you add much more to it. 72 x 90 inches is a standard twin size quilt. I made a 72 x 96 for a super single bed in a semi and it is too heavy for my Sis-in-law to drag in & out to wash. I used flannel for both sides and cotton batting.