The exact size of the universe is impossible to know. We know that the radius of the obversable universe is 13.8 billion light years as the universe is 13.8 billion years old according to science. It is called the observable universe as anything further away than this we couldn’t see yet as the universe is not old enough for the light to have traveled far enough to reach us!
So the volume of a sphere is
V = 4/3 πr^3
where r is radius
so plugging in 13.8 billion into that gives 11008.44 Billion light years cubed. 1.0415e+26km cubed
or 1041500000000000000000000000000Km cubed
That is a rough guess!
However! as the universe is constantly expanding this number is always getting bigger! Also we have no idea what is outside our observable universe. So it could be even bigger!