HELLO JAY

SInce you did not mention which galaxy let us assume it to be our home galaxy that is milky way galaxy
it is about 1000000 light years across
1 light year is defined as the distance travelled by light in vacuum in 1 year which comes approximately equal to 10 trillion kilometres.
so therefor just multiply 1000000 x 10 trillion which will give you the size of our milky way galaxy!!
HOPE IT HELPS!!