How much overhang do you want on each side?
Is the mattress 'normal' height or one of those 'extremely tall' mattresses?
Also consider the size of the person the quilt will cover. A taller, larger person might need a slightly wider/longer quilt than a smaller/shorter person.
http://www.sleepexperts.com/mattress-sizes
The link above shows the
top of a twin mattress to be 39 x 75.
http://www.quiltmaker.com/articles/S...dard_Bed_Sizes
Quiltmaker suggests 68" x 89".
Good Luck!