The electric field (E) between the two parallel plates of a capacitor will be uniform if (d is plate separation and A is area of plate)

Between parallel plates, the electric field is uniform because
$\to \mathrm{E}=\frac{\mathrm{\sigma }}{{\mathrm{\epsilon }}_0}$ is approximately constant because the distance between the plates is assumed to be small in comparison to the area of the plates
→ i.e., d < A