I will admit I was a little nervous going from 5 gal to 10 but I am glad I did. Filling two fermenters instead of one for only a marginal increase in time makes me happy.
I used a 24x24 grain that I picked up at my lhbs, it worked fine but it was not wide enough to clip onto the whole circumference of the keggle. It seems that the preferred method is to custom sew one from voile.
I doughed in with 10 gallons in the keggle, then I heated up 2.5 gal in a separate kettle to about 180, I lifted the grains out and placed them in the 2.5 gal, then lifted the grain again and placed it in a separate empty kettle, then poured the 2.5 gallons over it. Went back and fourth a couple times then dumped the water into the kettle. I then placed a colander in an empty kettle and placed the grain on it to drain. I could hardly say I have perfected this method but that is how I did it this time and it seemed to work fine.
Go for 10 gal, you won't regret it!