People always say and write, "As big as you can afford," but I've found that not to be entirely true. It really depends on how much time the bird spends in it. My large Congo Grey (right at 600 grams) has lived in an Avian Adventures Mediana cage for the past ten years. It's 34"x24" and has a play top. He uses the inside for breakfast and dinner (to contain the mess) and he sleeps in it. The only other extended time he spends in there is if I leave the house on errands (I work from home). The rest of the time, he's on his top play top, Atom hanging from the ceiling, on a manzantia play tree in the living room or in my office. On nice days, he goes in the outdoor aviary. Based on his living patterns, it would be a huge waste to have him in the absolute largest cage I could afford and a cage that large would take up so much uneeded space that could be better used for uncontained play space (like trees, play gyms, play stations, etc.).
My only suggestion is buy a good-quality cage. It will last you tons longer than a larger, inferior cage. I can still get parts for my cage model and it looks like it would last another ten years of use without any problems. It has held up extremely well (and I'm a clean freak that scrubs the dickens out of it daily). I have Avian Adventures (now owned by Midwest Metal) and Kings cages. I've been very impressed with the longevity of the cages and from the customer service provided. Good luck on your choice!
