Check your stove, before buying a new pot. Find the biggest pot you can borrow or somehow find, and fill it up with water. If you can boil a big volume of water, then measure it and see how much you can boil. Most people find that their stove just can't boil 6 gallons of wort, or sometimes even 4. Or 3.
A bigger pot won't help you if your stove can't do the job, so before investing in a new pot find out how much your stove is capable of.
I have a professional gas range, with one burner than could boil 6.25+ gallons of wort. Most electric stoves can't do that, but some can.