I have 600 v supply about 700 feet away from my motor that is 40 amps full load I need to find out the size of the wire needed

Dear sir, refer to cable catalogue to find out resistance per kilometer length of the cable. Same can be converted to feet. Now you have resistance of cable, current you are knowing... find out size of cable which gives you voltage drop not less than 2 to 3%.

Try:

Southwire - Delivers Power (http://www.southwire.com/ProductCatalog/voltdrop.jsp)

By entering your data, I get a #4 copper wire.

I have 600 volts at 20 amps 3 phase my load is 50 amp what size stepdown transformer do I need

A 20 amp load at 600 Volts 3 phase is 20,760 VA. Typically transformers need to be rated 25% higher than the load, so you would need at least a transformer rated 29,950 VA.