An 8" wheel is fine for a small Syrian
So a 7.5" probably will be too - but she might still grow a bit more. I think a 6.5" silent spinner is far too small and she won't be able to run properly or comfortably in it. She might try but a bigger one would be much better. What 12" one do you have? I have seen videos of a robo running in a 12" Wobust wheel! So as long as the wheel spins easily she might enjoy running in a 12" wheel. If it's a 12" silent "spinner" it may be a bit stiff or overwhelming.
You could just get a 28cm (11") trixie wheel. They cost about £9 from Amazon and will be suitable for her at any age or size.