Short answer. . . 18ga. to 16ga. speaker wire will generally suffice if you have 8 ohm speakers and you keep the volume level in a low to moderate (power) range.
The more power you try to push through your speakers, the more loss there will be in the speaker wire used to feed that power to the speaker. Additionally, a 4 ohm speaker will generally cause more current to flow for a given voltage output by the amplifier than an 8 ohm speaker. And the longer the wire length, the more power will be dissipated in the wire used to feed the audio signal to the speaker. Ideally you want Zero loss in the wire, but all wire has resistive, capacitive, and inductive losses, also known as impedance. A speaker has the same thing, but all wound up tightly in the speaker voice coil where you want most all of the audio power to do it's work. The longer the length of wire used to feed a speaker, of course, the more signal could be lost in the wire itself. This affects more than just overall "loudness" the speaker puts out. Why, because the "impedance" mentioned reacts differently at the various frequencies of audio signal passed along to the speaker by the wire supplying it, and even the speaker voice coil itself. See, lot's of things going on here!
So, generally 18ga for a low to average loudness level and 8 ohm speakers should be fine. To be honest, in my previous home I ran 18 gauge wire to some side speakers in a 7.1 set up and 14 gauge to some rear speakers of the same size (6" drivers with 1 1/2" tweeters) over a 30' to 35' run.. I measured audio signal level with both meter and oscilloscope at all speakers and at 4 different frequencies (60Hz, 1KHz, 5KHz, and 10KHz). The difference in signal level was negligible at low to moderate audio signal levels from the amplifier. When the volume was cranked up to very fairly high levels that I would rarely, if ever, use; only then did I see significant degradation in the audio at each speaker. In my new home for our living room system I just ran 18ga wire for side and rear speakers. For my more serious room in the lower level, I will run 14ga wire, but that's just me.