I haven't needed bigger than a 5 gallon one in all my years of all grain brewing.
Rarely have I ever had a grain bill that exceeded 13 pounds of grain that my 5 gallon cooler holds at a standard water to grain ratio we use.
I get beat up for daring to post this, SCANDELOUS idea, but I've used my 5 gallon cooler for over 5 years, on 5 gallon all grain batches and have never had a problem. At 1.25 it actually holds nearly 13 pounds of grain. Since most of my grainbills run between 10 and 14 pounds, I have no problem. for 14 pounds I just use about a 1.15 quarts/pound mash dillution. Or for 15pounds I go with a 1 quart/gallon.
14 pounds of 2-row in a 5 gallon batch gives you a 1.076 OG so it's not a small beer.
I rarely need a larger mash tun. If I'm making a bigger beer I'm often doing a 2.5 gallon batch instead of a 5 gallon one and those 14 pounds would then make a beer with an og of 1.151..... If I do want to make a 5 gallon batch of a big beer, or a larger than 5 gallon batch, I have a TON of homebrewing buddies, with all sorts of systems and sizes of mash tuns, and they'd let me use their geer. and if it's a barleywine or something like that, we're all brewing together then anyway, or hanging out.
I really don't have the space for a 10 gallon cooler, and like I said, in 5 years of using my 5 I've never had the need to go with a bigger one for more than a 1 shot deal. I maybe brew 1 huge beer a year anyway.
Other folks have other opinions, but in truth, you can make a lot of beer in a 5 gallon cooler.
If you wanna be realistic and practical and it not just be about the biggest toys, you need to ask yourself a couple of questions.
1) Am I ever going to brew larger than 5 gallon batches?
2) Are the majority if not all of my batches going to have an og higher than 1.076 (14 pounds of grain?)
3) Do I have the money or the storage space for a larger one?
It really comes down to those questions. In my case all three of those were no.
You could also add a fourth question- If I need to brew bigger once in awhile can I lay my hands on a bigger cooler. My Answer to that was yes.
So to me, a larger cooler was overkill, in light of living in a loft and not having much storage, nor having a realistic need for a larger one.