Honestly, start small and work your way up. When I first started my quilts were quilted together and they looked awful LOL My cats and dogs had tons of "beds" and I passed on others to my friends. I started with 36x36 - it's workable and they are fast and easy. Once you get the hang of it, then you can start going bigger and bigger.
I've been quilting for over 15 years, and while I have made some fabulous big quilts, my preference is still something around 48x48 inches.