Dimensions:
Each “spot” is 65mm outside diameter (about 2.55”), with a 51.5mm diameter pocket (a bit over 2”) to hold the base of your Amiibo (or whatever else you want to put there).
The 3-tier stands measure 180mm front to back (a hair over 7”). The 4-tier stands measure 206mm front to back (about 8.125”).
To figure out how wide your display will be, count up the maximum number of “spots” wide your display will be (the second row is the widest) and multiply it by 65mm (2.55”) to get the total width of your display.
Alternately, measure your opening, and divide by 65mm or 2.55”, and round down to the nearest whole number to know how many stands you can fit in that opening.