  Average magnetic field measurements are presented for 62 M-dwarf members of
the Pleiades open cluster, derived from Zeeman-enhanced Fe I lines in the
H-band. An MCMC methodology was employed to model magnetic filling factors
using SDSS-IV APOGEE high-resolution spectra, along with the radiative transfer
code SYNMAST, MARCS stellar atmosphere models, and the APOGEE DR17 spectral
line list. There is a positive correlation between mean magnetic fields and
stellar rotation, with slow-rotator stars (Rossby number, Ro$>$0.13) exhibiting
a steeper slope than rapid-rotators (Ro$<$0.13). However, the latter sample
still shows a positive trend between Ro and magnetic fields, which is given by
$<$B$>$ = 1604 $\times$ Ro$^{-0.20}$. The derived stellar radii, when compared
with physical isochrones, show that on average, our sample shows radius
inflation, with median enhanced radii ranging from +3.0$\%$ to +7.0$\%$,
depending on the model. There is a positive correlation between magnetic field
strength and radius inflation, as well as with stellar spot coverage,
correlations that together indicate that stellar spot-filling factors generated
by strong magnetic fields might be the mechanism that drives radius inflation
in these stars. We also compare our derived magnetic fields with chromospheric
emission lines (H$\alpha$, H$\beta$ and Ca II K), as well as with X-ray and
H$\alpha$ to bolometric luminosity ratios, and find that stars with higher
chromospheric and coronal activity tend to be more magnetic.
