I use a old Gitzo 1548 for my 600mm FL. But new and lighter options are now available.
My 1548 folded is just under 23.25 inches(59 cm) and weighs 3 Kgs. Load capacity 66 pounds ( 30 Kgs)
My worst case, weight of equipment to be mounted on the tripod, would be under 10- 15 Kgs
Folded length is important so that I can fit the tripod into a suitcase for travel. Hence I prefer folded length under 24 inches.