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

f=gm1m2/d^2 it d

f is force g is gravitaiton constant, m1 is mass of first object, m2 is mass of second object. d is distance between two object