You actually want the aging/bright tank to be smaller than your primary. Closer to the actual batch size in volume. So if you brewed a 5 gallon batch (or typically will) then just get a 5 gallon carboy...
BUT, as already mentioned, except in a few cases many of us are dropping the racking to another vessel. I'm getting much better brews since going to longer secondary times (on the yeast cake)... I've gone 6 weeks so far, but have a couple of bigger brews that I'm planning on leaving on the yeast cake for 6-8 weeks. I'm pretty sure the actual fermentation will be complete, so I'll just taste test it every week (or two) after about 4-5 weeks to see where it's at. WHEN it's ready for the next step, that's when I'll rack it. I'm considering aging on some oak cubes, which I'll probably do in Sanke kegs. Once that's done, IF I want to age for a few more months, I'll probably rack again, or I'll make sure I can pull the oak cubes out of the keg, and then just leave the brew in there (that's the more likely scenario)...
Properly using either an auto-siphon, or racking cane, will get you the best chance of no oxidation issues. But, if you're not actually adding flavor elements that are best done off the yeast, or plan on an extended aging time frame, just leave the brew in primary until it's ready for bottles/kegs.