  The dust grain size distribution (GSD) likely varies significantly across
different star-forming environments in the Universe, but the overall impact of
this variation on star formation remains unclear. This ambiguity arises because
the GSD interacts non-linearly with processes like heating/cooling, radiation,
and chemistry, which have competing effects and different environmental
dependencies. In this study, we investigate the effects of GSD variation on the
thermochemistry and evolution of giant molecular clouds (GMCs). To achieve
this, we conducted radiation-dust-magnetohydrodynamic simulations spanning a
range of cloud masses and grain sizes, which explicitly incorporate the
dynamics of dust grains within the full-physics framework of the STARFORGE
project. We find that differences in grain size significantly alter the
thermochemistry of GMCs. Specifically, we show that the leading-order effect is
that larger grains, under fixed dust mass and dust-to-gas ratio conditions,
result in lower dust opacities. This reduced opacity permits ISRF photons to
penetrate more deeply and allows internal radiation field photons to permeate
more extensively into the cloud, resulting in rapid gas heating and the
inhibition of star formation. We find that star formation efficiency is highly
sensitive to grain size, with an order of magnitude reduction in efficiency
when grain size increases from 0.1 $\rm\mu m$ to 10 $\rm\mu m$. Additionally,
we note that warmer gas suppresses the formation of low-mass stars. Moreover,
as a consequence of the decreased opacities, we observe a greater proportion of
gas residing in diffuse ionized structures.
