My run is 300 feet from the generator shed to the transfer switch, then into the house and the distribution panel. I was concerned about voltage drop at maximum power usage (50A - 240VAC). I used AWG #1 gage for the conductors and less for the neutral and ground (IIRC AWG #4). I do not regret spending the extra money.
Part of the #1 AWG justification was the permitting process for the tool shed with 100 amp service. I ran a line 50 feet from the genereator shed to hit the 250 foot line running from the tool shed to the house service panel. Thus, 250 + 50 = 300 feet run.
My suggestion is spend as much as you can on wire, over build if you can, do it right, and you'll never have to worry about voltage drops, poor performance, insulation fire, etc.