We are redoing the majority of the supply in a house we are remodeling. Is there any code or standard on using pex when attaching the water heater. The supply comes in the house as copper. Can we convert it after the meter to pex or do we have to wait until after the water heater? If so, how far after the water heater? If we can convert it before the water heater, can we use 1/2" pex?

You can convert to PEX right after the meter. Does your meter have a jumper cable on it for the house electric? If so you will need to arrange a new grounding for the home after switching to PEX.

Can I use 1/2 inch pex from the meter to the water heater or does it have to be 3/4 pex?

If your main at the meter is 3/4" then stick with that as a good trunk line. You can drop down to 1/2 " for your taps to supply fixtures.