Can I use 24 gauge speaker cable?

My answer is YES.  You can use 24ga. wire on runs of 50+ feet. I have done so through 5 rooms of my parents house.  Pushed with a Wollensak  reel to reel  mono tape player.  Strictly soft background music.  Small oval 8ohm speakers.  Ran for years.  I replace the Wollensak with a mono fm radio.  Knight? KLH? receiver.  Hey! given me a break.  This was back in the 70's.

Yes with agreement to comments about wire impedance and sonic differences.  Let me remind you of this site = HIGHEND+   Thus should you push 300 watts to your MagnaPlanars, much wattage will be dissipated in the resistance of the 22ga. wire.  Heating up the wire. 
Also there are significant Sonic advantages of implementing thicker wire gauges of different materials and technologies.  I confirmed this to myself when I rebuilt a friend's 3 way Sansui speaker from the 70's.  I just by replacing the solid 22ish ga. wire used with at the time "Monster" cable.  8-10ga.?  The longest run of the three was 14" from the crossover to tweeter.  Wow.  What a difference.  Even when driven by my little Nakamichi receiver on my workbench. Crossover was next.  After which I had to get them out of my shop!  Sonically they were approaching new DCM's and Alon's in the sound room. 

Here's a very old table that's been used to gauge speaker wire applications and it's not just for silver speaker wire, which this site happens to sell:http://tempoelectric.com/cables_speaker-cables.htm

You'll see this table used by a lot of old audio sites and it's probably still relevant unless something's been reinvented.

All the best,
Nonoise

Good lord, who comes up with these thread ideas?  Okay, I guess we know who, but still...

It is a function of length and current.  So it should be great to run a tweeter two feet away.

Seriously,  How far away do you want to put the amp from the speakers?  How big is your power amp in watts?  What speakers?

A simple answer is a smaller gage is better.  When I hook up even small built in wall speakers we run 14 gage in the walls.

Some interesting reading on the topic: https://www.biline.ca/audio_critic/mags/The_Audio_Critic_16_r.pdf

"The Wire and Cable Scene: Facts, Fictions, and Frauds Part II" starts on page 51. The article is almost thirty years old but the science hasn’t changed.If you want to cut to the chase, "So what’s the best thing to do" is on page 56.

Here is another interesting article on the topic: http://www.firstwatt.com/pdf/art_spkr_cable.pdf