I think what your god-daughter wants is that Martha Stewart flower spacer cake (the original design was done with hydrangeas years ago). Ron Ben-Israel does this design frequently with his gorgeous sugar flowers, so there might be some photos on his site or that you can google for inspiration about dimension and proportion.
Is it because of the number of servings that she wants a 12/10/8 real cake? I'm not sure that gives you enough room for whatever kind of flowers she's using, and that brings me to my next thought:
A 5" tall spacer is a HUGE gap for flowers (and major cost for the flowers). Do you know what kind of flowers she's using? Hydrangeas are more "poufy" than something like roses so how much of the flowers stick out beyond the cake is really all about what kind of flowers she's using. Is the florist going to insert the flowers on the day of, at the wedding venue?
Panini has the right idea in having the first styrofoam spacer be 6" wide to support the 10" real cake above it. And a 4" wide spacer for the next level.
I have some concerns about how the florist is going to put the flowers in the styrofoam - the floral stems need to be somewhat sturdy to be pushed through styro, and if you cover the styro with fondant (so the styro stays intact and doesn't break up into small bits while being dismantled. And you need to secure the spacers to the real cake underneath (you need to have supports in the real cake, not the styro) which also has to be done onsite. (use long bamboo skewers)
For what it's worth, when I've done this design, I've used a center pole stand and had the florist use oasis floral foam rings, which they put into plastic ring dishes. When my last client didn't bring back all the parts to this stand, I had to improvise going forward. So now, if someone asks, I'm using the clear plastic plates and push-in pillars (they kind of look like spikes, but they're clear so they won't show) and then I build the cake onsite. The florist puts the fresh flowers in between the pillars and the florals usually cover the pillars. If you go this route, you will need to buy 7" pillars or 9" pillars, and a 9" plate (to put the 10" cake on) and a 6" plate (for the 8" cake). Put the cakes on small cake drums so you can handle them easily, and bring heavy duty tape to secure the cake to the plate. When I leave the shop, the bottom tier is on a cake drum, and the plate/pillar is already in the cake. Then the middle tier (also on a cake drum) is in another box, with the plate/pillar already in. And the top tier in a third box. You arrive, put the bottom tier in place, put loops of tape on the plastic plates, place the middle tier, then place the top tier and let the florist fill in the spaces.
Make sure the cake table is level.