I just started on this recipe 4 days ago and I might be able to help out a little. I am a long time reader and first time poster but here goes-
I started with 3 gallons of water in a 5 gallon kettle. I thought I was going to have an issue with adding 10 pounds of sour cherries, but I was wrong. There was a surprising amount of room left in the kettle, especially after 45 minutes of boiling. I would estimate 7-8 inches after adding cherries. I used cans of Oregon brand tart cherries in water (11 x 14.5 oz), drained, and dumped into a grain bag. I crushed with clean hands.
My wort temp was about 223ºF at the 45 min mark. I killed the heat and added the bag of cherries mixed with finishing hops. The temp dropped down to 184ºF. I did not really believe it so I checked my digital with my trusty back-up thermometer. It was accurate. I kept the heat off until the temp dropped down to 165ºF and then slowly added/removed heat to keep it in range.
When it came time to pour it into the fermenter, I removed the bag of hops and cherries to another kettle. I dumped the hot wort over ice and filled to the 5 gallon line with cool tap water. After stirring, the temp was 85ºF. Again, I double checked (10 lbs of cherries cost me $45 after all). I simply waited for the temp to drop to 80ºF and pitched the yeast. After, I added the cherry bag again, sealed it up, and waited.
Fermentation started 48 hrs later (which worried me) but when it did, WOW! I am using a standard, cookie-cutter, brew store-purchased plastic primary and there is plenty of room. I see no issue with using a glass carboy either if that is your thing.
Hope that helps and good luck,