A total of 6' of roost would be good, but your birds if sitting close together will not actually *occupy* more than about 3-3.5' of roost, so if you are very tight on space you may well be able to get by with something towards those smaller numbers, it just gives you less margin for error if there are some birds that don't get along well.
Because 2 of the birds will be silkies, they'll need the roost low enough they can hop up onto it. This may conflict with wanting to have a higher roost if you are hoping your phoenix will develop a nice long tail, but, whatcha gonna do. One option would be to make a ladder-style roost, perhaps that would be the best idea in this situation?
LindsayB, unless you are in a very cold part of TX with a very open drafty cold coop, you will likely be fine with just a 2" wide roost; although in general IMO it is *better* to use a wider roost, even in hot climates.
Good luck, have fun,
Pat