I used to brew in my 7g canning crock. It was MORE THAN SUFFICIENT! Hell, I brewed a 6.5 gallon batch in it - without boil-overs. You guys don't need nearly 3 gallons of headspace. 8 gallons will be fine for your 5-5.5 gallon batches. The only time I have ever had boil-overs is when I left the pot alone before it had settled into its full-rolling boil. I guess people are getting too lazy to stand around their pot while it starts to boil nowadays, and adjust to maintain a full boil without boil-over.