True, we can use long double; if double range is not enough.

double = 8 bytes.
long double = 10 bytes.

True,
float = 4 bytes.
double = 8 bytes.

True, When a function is declared inside the source file, that function(local function) get a priority than the extern function. So there is no need to declare a function as extern inside the same source file.

True, we can find the size of short integer and long integer using the sizeof() operator.
Example:

#include<stdio.h>
int main()
{
    short int i = 10;
    long int j = 10;
    printf("short int is %d bytes.,\nlong int is %d bytes.",
            sizeof(i),sizeof(j));
    return 0;
}

Output:
short int is 2 bytes.
long int is 4 bytes.

False, The range of double is -1.7e+308 to 1.7e+308.