My 148' LoG can receive longwave BCB stations with no preamp when propagation cooperates, but those stations are using extremely high-wattage transmitters. I am not really into NDB monitoring, but I have noted some when spinning the VFO down there.
An "ideal" start for a fractional loop is probably around 15% of the the desired wavelength.
Let us look at the bottom of the AM BCB band for an idea:
530KHz = ~565m * 0.15 = 277.5m
So around a 900ft loop there. Proximity to ground will lower resonance by a few percent, but we are dealing with such large loop sizes now, it is a "whatever" IMO.
If we disregard noise, ~1000ft+ would be a good start for an "ideal" low-frequency BoG. However, low frequencies are typically swamped by atmospheric noise, so you simply need an antenna with enough capture area for the receiver to detect the desired signal. Above that is basically just added gain, and it is about SNR instead of absolute gain.
TLDR? For longwave and lower, deploy the largest LoG you can.

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About common-mode noise, let us start with a very basic test assuming we are talking about a typical residential deployment. Tune your desired frequency with an antenna in the house, even if it is just a few feet of wire. Now hookup your outside antenna. Regardless of the signal level, do you hear the *same* noise? If so, you might have common-mode noise. If common mode is suspected, to further evaluate if the noise is being picked up by the antenna or the feedline, you would need to evaluate if the same noise is present at your antenna location. For example, by using a portable receiver.
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Your Nooelec balun still is acting as a impedance transformer, though it is so small I am not sure what effective impedance transformation it is doing at such low frequencies. Hmmm. Considering a 60' loop is going to resemble almost an electrical short at longwave and lower, an 9:1 might actually work better if you turned it around at the antenna feedpoint. YMMV.