Seriously, I don't think I'll ever have the need for a 12 footer. Don't get me wrong, I'd love to roll up next to a brand new V21 in my ol '72 and open up the prettiest 12 foot spread as the next guy, but really my 6 footer does the job well. The best way to learn is to watch somebody and practice. I use the "teeth" method, meaning I use my teeth to hold part of the net. I find this method to be the best, but what do I know?
I wouldn't recommend starting with a 12 footer either, but I'm sure I don't have to tell you that. Start anywhere between 3 - 6 ft, whatever you have available or what you feel most comfortable with. I've save a ton of $$ by using my net. Heck the only bait I use on the Lake is shad that I catch in my net!
I love cast nets! All I need is an ice maker! So I don't have to buy ice! Course I do rob my freezer's automatic ice maker from time to time!