I don't have an adult retic but I have been looking at cages to order. I've been leaning towards an 8x3 for my female and either 5x3 or 6x3 for my male. My female is from a small female that has topped out at 11ft so I don't expect her to get too large.
But I'm interested in the answer to this too. I've asked/searched before on other forums and it seems between the 6x3 or 8x3 for females and generally smaller for the males.