I am going to install a dedicated circuit to wall mount a/c, with the following details; single receptacle, 40' run, from distribution panel direct to outlet box, Have not acquired a/c unit, when I do, I will install adequate amperage breaker according to manufacturer's cut sheet. In the meantime I need to purchase conductor and install while wall is open. What would be the proper romex type cable to use? Thank you

#10 would be appropriate for a typical 30A circuit. You can run 10/2NM.
I have to say, unless this unit will be HUGE, you will not likely need 30A

#10 would be appropriate for a typical 30A circuit. You can run 10/2NM.
I have to say, unless this unit will be HUGE, you will not likely need 30A
stanfortyman , thanks for prompt answer, probably overkill w/ the 30, 20AMP is most likely, would 12/2 nm be appropriate? Thanks

Yes, and since this will be a dedicated circuit you can convert it to a standard 120v/20a circuit later if you needed.