Not really anything to design, it's fractional wavelength, electrically short -- just make a loop as big as you can afford, then match that to 50 ohms or whatever with a suitable transformer, or coupled loop, or matching network. Make it resonant to get higher gain at very narrow bandwidth, or ignore the reactance to get very low gain (and radiation resistance) over whatever bandwidth you like.
Conductor size: also as much as you can afford. Litz an option (pretty coarse at these frequencies, a bundle/multiconductor cable of stranded wire would do quite nicely). Reduces electrical resistance, making radiation resistance a larger part of the total (so, increasing SNR; but still much, much smaller than reactance, unless you tune it out).
Use balanced designs, or shielding, to give immunity from local fields (typically of the opposite type, i.e. an inductive loop with E-field immunity, or electrically-short dipole with H-field immunity).
Tim