AI'm doing my first cake with square tiers and had planned on doing 12", 10", 8", and 6" tiers. But someone told me the other day that they'd recommend doing 4" differences between tiers. Is there a standard for square tier sizing? The cake is king to be iced in SMBC and have a black ribbon. No flowers, etc. on the tiers.

Thanks for your help.

You determine the tiers according to the number of servings required.

There is absolutely no hard and fast rule.  There is also absolutely no sense in baking 6-10-14-18 inch tiers just because of some theoretical idea.

It all depends on the amount of ledge you want between each tier and the one below it.  A two inch difference gives a sleek narrow 1" ledge around the cake - clean lines and not too much room for extras.  A four inch difference gives a two inch wide ledge all around.  This is nice if you have a floral display or something that needs it.

Personally I prefer the wider look, but it is all what look you want.