just for the fun, you can do nmr with earth field... some ground topology analysis use nmr at earth field, so the sample is at kilometer scale :)

NMR is a 3 dimensional action, and so occurs in a volume rather than over an area. The active volume of an NMR Spectrometer is equal to the length of the probe's RF coil multiplied by the cross-sectional area of the inside of the sample tube.

The question is not clear but if you mean the sample, the rf frequency on the sample you can see this here: http://qa.nmrwiki.org/question/38/what-is-the-optimal-sample-length-with-shigemi-tubes

Field is proportional to frequency, so it has the same "areas" (whatever you mean by "area"). Diameter of coils effects on sensitivity because you have more sample producing the signal of free induction decay.