Nathan, the short answer is that for a run that long you should probably use at least 14ga. A more detailed discussion of speaker wire gauges and related matters is found in
this excellent explanation by a veteran audio professional.
What may cause a problem when the wire is too thin is the fact that two resistances in series(e.g., the resistances of the connecting speaker wire and the wire in the speaker voice coils)form a voltage divider. Briefly, each wire "uses" the proportion of the output voltage from the amplifier that its resistance bears to the total resistance of the two wires. Since speakers typically vary fairly widely in resistance(impedance)with frequency, as the speaker impedance fluctuates, the voltage that it uses in comparison to that of the connecting wire also fluctuates. When the resistance of a too thin speaker wire is too close to that of the speaker itself, the variable voltage that the speaker wire absorbs as its impedance fluctuates can cause an audible frequency response irregularity. For example, if impedance was 4 ohms in the bass area and 15 ohms in the treble, the treble would be emphasized. Mr. Russell suggests in the cited article that the resistance of the speaker wire be no more than about 5% of the speaker impedance.