Is our universe the size of a grain of sand?

No

Explanation:

Our universe is not the size of a grain of sand. A grain of sand is approximately 2 millimeters while the size of the universe is approximate 93 billion light years in diameter and recently estimated to be 250 times 93 billion light years which is at least 7 trillion light years.

Light-year is a unit of length used to show astronomical distances.

1 light-year is exactly  9460730472580800 meters.

Comparing the size of a grain of sand to that of the universe,

2 millimeters = 0.002 meters and

9460730472580800 meters =  9.460730472580800 e+18

r=rho l/a where a is the area, l is the length and rho is resistivity so the greater the area the lesser the resistance thats why the earth pin is thicker bcos current follow the shortest path, so less resistive .