i know how you feel, the first year of quilting my quits would grow and grow, i didn't seem to be able to figure out when to stop...my daughter's wedding quilt is 12 feet x 14 feet...a huge quilt...my son's grew even more...i have spent the past few years really working on keeping my quilts within boundaries!!!
one way that has worked for me is...
if i'm planning a queen size quilt i start with the exact top measurement of the queen mattress, and make my design fit that measurement exactly, then i decide how to frame it and borders to give it the hang/ tuck measurements i want it to have...every quilt i make i have to really measure and plan each step of the way, otherwise i get very carried away...i tell everyone i am very (un-disaplined) every quilt is a lesson in control :)