Anyone have a clue what size this cake is? 6" & 8"? 

https://s-media-cache-ak0.pinimg.com/236x/ae/b1/05/aeb105bea7d37160d2a773bac42c0f0b.jpg

Looks small. I would say either and 8" and 6" or a 6" and 4"

:)

Looks like a 6 & 5 to me.  

Judging by the looks of the cake stant it is on It probably is 6/8 BUT.........more importantly is how many you need to serve.  

thanks all! she wants to serve about 20-25. 

That cake stand looks pretty small,  4/6 perhaps?  Anyway for the number of servings needed, you really only need a one tier 8 or 9.

I would guess 4" inch and a 6"  judging by the marshmallows tower next to it. If she wants to feed 20 to 25 people though I would definitely make it in a 6 & 8" inch. The six-inch typically will only feed 10 - 12 people if cut traditional wedding cake style  1 x 2 inch slices.