Flying saucer size for a Roborovski

[nefosaur]

Hey hamster keeper friends. I just adopted 2 (separately housed) Roborovski hamsters who are about 4 months old. The setups they came with need upgrading and they are currently using 4 inch wheels which I have read are too small. I'm thinking of replacing them with flying saucers. Could you tell me what size flying saucer is needed for a Roborovski? I heard one Youtuber say you need to go one size up than the minimum wheel size for your species of hamster, but I cannot find anything else on this.
And just to confirm, the minimum wheel size for a Robo is 6.5 inches, yeah? So should I get a 6.5 inch flying saucer, or perhaps an 8 inch?
Thanks