  Structural properties of cluster galaxies during their peak formation epoch,
$z \sim 2-4$ provide key information on whether and how environment affects
galaxy formation and evolution. Based on deep HST/WFC3 imaging towards the
z=2.51 cluster, J1001, we explore environmental effects on the structure, color
gradients, and stellar populations of a statistical sample of cluster SFGs. We
find that the cluster SFGs are on average smaller than their field
counterparts. This difference is most pronounced at the high-mass end
($M_{\star} > 10^{10.5} M_{\odot}$) with nearly all of them lying below the
mass-size relation of field galaxies. The high-mass cluster SFGs are also
generally old with a steep negative color gradient, indicating an early
formation time likely associated with strong dissipative collapse. For low-mass
cluster SFGs, we unveil a population of compact galaxies with steep positive
color gradients that are not seen in the field. This suggests that the low-mass
compact cluster SFGs may have already experienced strong environmental effects,
e.g., tidal/ram pressure stripping, in this young cluster. These results
provide evidence on the environmental effects at work in the earliest formed
clusters with different roles in the formation of low and high-mass galaxies.
