The bigger the better, as the radiation angle will drop as the loop gets bigger (encloses more space). See www.cebik.com
where W4RNL has many excellent antenna articles, including some on horizontal loops.
My experience with horizontal loops makes them my favorite choice for a nearly-omnidirectional all-band antenna, and my next one will include a tuner mounted at the feedpoint, or connected to the feedpoint by ladder line.
My last loop zigged and zagged a bit through tree branches, was only 20-25' above ground, and had a fundamental resonance around 5 mHz, but it worked very well, better on successively higher bands, and beat my vertical dipole all hollow on 10m. I'm replacing the fan dipole I just lost in the ice storm with a horizontal loop! (just as soon as the ice melts a bit)
73 & best rx de kt8k - Tim