Congrats Thejoie on becoming a new to your grey owner, you must be so excited
I have to agree with Janet, go ahead and purchase a bigger cage and have it ready for when you bring this grey home for one more new thing won't make that much difference. He will be coming to a new to him home, cage, people and completely new to him surroundings, just talk to him and let him settle into his new home and cage.
Bigger is always better, they need room to be able to open their wings fully and not touch the sides so if you have the space for a larger cage than he now occupies then by all means get him a large one.