I have an 11 gallon pot, and the 5 gallon paint strainer bags at the hardware store are plenty big enough to brew a BIAB batch up to about 13lbs of grains. They are really the most common bag used for peeps with 10 gallon pots. The bag sits about 3" above the bottom of the pot and 6.8-7.2 gallons of pre-boil volume for a 5-5.5 gallon batch covers the grain bed by plenty.
You don't need a 10 gallon bag to fit a 10 gallon pot. The 5 gallon should work just fine unless your pot is unusually wide in diameter.
Primary #1 - Midnight Ryeder (Midnight Wheat and Rye)
Primary #2 - Florida Weiss
Primary #3 - Kane-DOH APA (Honey Citra APA)
Secondary #1 - Downtown Flanders Brown (brewed August 2012)
Keg #1 - Raspberry Florida Weiss
Keg #2 - Cinnamon Raisin Cider
Keg #3 - NONE!
Bottled - NONE!