I would recommend a fabulous book that I own-you can get it cheap from Amazon, but you could probably find it at a library, too. It's called Complete Guide to Quilting, published by Better Homes and Gardens. It has a couple of cheat sheets with sizes that would probably be really helpful.
There are so many variables for how big to make a quilt, but the most basic would be to measure the top of the mattress plus the three sides (excluding the headboard end).
Pick a size for your pattern squares (6", 10", whatever), and then do the math or make a drawing to figure out how many squares will cover the bed. And don't forget that you can add one or more borders around the outside if you need just a couple of extra inches.
But honestly, don't the math and the planning bog you down. It should still be fun!