If you spend any time with a group of quilters, you'll hear the "rules" . When you are just starting out you google, you ask the professionals, you turn to the blue ribbon winners for advice and you will hear : Things You Must Follow To Be A Real Quilter.
At the risk of being tsk'd tsk'd, I say take the rules, see if they work, and if they don't, toss 'em. nobody will take your rotary cutter away, I promise.
1. Full size, Queen, Twin?
There is the assumption that when making a quilt, it must go on a bed. It must drape nicely over the sides and allow for pillows.
Pfooey on that. Maybe I want a quilt to snuggle on the couch and it needs to be just long enough for your favorite couch potato's toes to be covered and wide enough to accommodate two kids and a few small animals. ( He requested that it be exactly this size. On a side note, It's hard to measure around cats. They keep grabbing the tape)
Or maybe I don't even want it to be a rectangle at all. How about a hexagon shape meant to lay on the floor for baby to crawl on ?
You can make any size or shape you want, from teensy dollhouse size to great ginormous museum pieces. Make it round, or square, practically sized for the bed if you like , or absolutely impractical polygonish and enjoy the quirkiness of it.