Gauss's Law :

The total electric flux over a closed surface in vacuum is equal to 1/ε₀ the total charge enclosed by the surface.

if q is the charge enclosed by surface S, The total electric flux through the surface S is Φ=q/ε₀

while measuring the electric flux, surface area is more important than its volume or its size.

Example:

let us consider a closed surface S encloses a point charges +q1,-q2,+q3 and -q4, the total charge enclosed is q= q1-q2+q3-q4

∴electric flux across S will be Φ=q/ε₀=(q1-q2+q3-q4)/ε₀

The total electric flux is zero if no charge is enclosed by the surface.

conclusion:

The closed surface chosen for application of Gauss's law is called as Gaussian surface. It can be of any shape or size