Share 'Compact 50/34 crankset for Cyclocross?'

I have been doing a lot of internet surfing tonight to figure out what crankset to purchase. I guess my question is what chainring size to get. You have everything from 36/46, 38/46 & 50/34. What is the standard size? What about a compact 50/34? I am looking at doing some races, but will do some gravel fireroads and singletrack.Thanks