When to cast size_t

I'm a little confused as how to use size_t when other data types like int, unsigned long int and unsigned long long int are present in a program. I try to illustrate my confusion minimally. Imagine a program where I use

void *calloc(size_t nmemb, size_t size)

to allocate an array (one- or multidimensional). Let the call to calloc() be dependent on nrow and sizeof(long int). sizeof(long int) is obviously fine because it returns size_t. But let nrow be such that it needs to have type unsigned long int. What do I do in such a case? Do I cast nrow in the call to calloc() from unsigned long int to size_t?

Another case would be

char *fgets(char *s, int size, FILE *stream)

fgets() expects type int as its second parameter. But what if I pass it an array, let's say save, as it's first parameter and use sizeof(save) to pass it the size of the array? Do I cast the call to sizeof() to int? That would be dangerous since int isn't guaranteed to hold all possible returns from sizeof().

What should I do in these two cases? Cast, or just ignore possible warnings from tools such as splint?