added code to depict use of set.size()

set/size.md

@@ -0,0 +1,23 @@
+# size
+
+**Description :**
+    This method is used to return the size of set. The return type is size_t (quite similar to unsigned integer), which overflows when a bigger number is subtracted from it.
+
+**Example :**
+```cpp
+//Run Code To Demonstrate use of set.size()
+#include<iostream>
+#include<set>
+
+int main(){
+    // Create a set object holding integers
+    std::set<int> mySet {1,2,3,4,-5};
+
+    std::cout << "Size of my set is : " << mySet.size() << std::endl;
+
+    return 0;
+}
+
+```
+
+**[Run Code](https://ideone.com/guEojH)**