You can also make a memory quilt for a  youth or crib bed as your template.  A standard crib mattress is approximately 54 1/2″ x 30 1/2″.  I would only make is just a few inches bigger if I was making it for a crib.  On the other hand if you are making a youth bed memory quilt, then by all means add the 12″ for your over hang. Two sides to a bed would have you adding 24″ to the  30 1/2″=54 1/2 ” by 12″ to 54 1/2=67 1/2 “.  So your finished size would be 54 1/2″ x 67 1/2″    A memory wall hanging quilt can be how ever big you want it.  A nice size to work with for a memory wall hanging is the finished  youth bed size 54 1/2″ x 67 1/2″.  It is large enough to not look like it is lost on the wall, but at the same time, it doesn’t over power the wall either.