char *variable;
int size = 120, tmpsize = 0;

variable = (char *)malloc(size*sizeof(char));

while ( ... ) {
	//Realloc memory
	tmpsize += 40;
	if (tmpsize > size) {
		size += 120;
		variable = (char *)realloc(variable, size*sizeof(char));
	}

	strcat(variable,"your string to add with a fixed size");
}

Realloc memory at every X steps. Useful when allocating fixed number of items (in this case chars).

Memory reallocation

C