The cubby holes are 4-1/2"x4-1/2" each.

I see two flaws in this wine storage design. The cubby holes are a little too large and standard 3.5" diameter wine bottles roll around (as seen in the photo). Champagne and Bordeaux are 4.0" diameter so they are better sized, but most other regular wine bottles are only 3.5". The other problem is more serious. The rack is located right on top of the frig where there is often heat which is of course bad for the wine. The cubby's do not seem to be well ventilated either, compounding the problem. There are other better designs for holding the wine bottles.