What are the dimensions of US outlet holes?
Let me introduce you to the wonderful world of Google Image search:

nema 5-15 dimensions

It's a great way to find drawings and other visual information.

Here's a drawing pulled from the search results:



Powercord CN

Dimiensions are millimeters with a comma used instead of a decimal point.

NEMA Website

Following some image searches and regular searches I also came across the official NEMA Website

This NEMA WD 6 - Non-Locking Plugs and Receptacles - Excerpt PDF should have the information you want.