Well, it depends on the size of your batch, of course!
6 gallons would be the minimum for a 5 gallon batch. 5 gallons are fine for a 4.8 gallon secondary though.
Personally, I like the glass 6.5ers for primarys because they're easier to scrub clean if you've got gunk in them.
In Process: Mango Beer, Homebrewers Pale Ale
Bottled/Kegged:Spicy Light Rye, Rice-adjunct Pale Ale, Mild Bourbon Porter, Roasty Stout, Basic Light Mead, Bourbon County Stout Clone
Up Next: Berlinerweiss, Chocolate Raspberry Ale, and American IPA