What are the standard dimensions for fridges in the US?
There appears to be no real standards.  I found this website which has a table with dimensions:  http://www.rempros.com/dimensions/refrigerator_sizes.html
But the best thing I found is just Home Depots search filters.  From the images below you can deduce some things.  There are a total of 1,161 items, these are the percentages each dimension make up of that total.
Common Widths:

22 in - 3.6%
24 in - 11.6%
28 in - 10.1%
30 in - 9.9%
33 in - 12%
36 in - 30.8%

Common Depths (tolerance up to +1.5 inches):

22 in - 4.7%
24 in - 10%
26 in - 9.7%
28 in - 15.6%
30 in - 20%
32 in - 23.4%
34 in - 8.1%