Hello Jodie, Depends on how many you're serving...party servings are about twice the size of wedding servings, Rich fruit cake servings are smaller than sponge servings. I recently put a 6" hexagon on top of an 8" round. (only because I only have the one hexagon tin!!!!)
I always thought that it should be a 2" gap ie; 6,8,10,12...but some people on here do 6,9,12...I think it depends on servings needed, what tins you have, and what you feel like doing!
If you are stacking for the first time there are some good youtube tutorials on using dowels/straws. I would definitely use sturdy plastic dowels for a rich fruit.
Have fun experimenting