format ‘%d’ expects argument of type ‘int’, but argument 2 has type ‘size_t’ [-Wformat]

Did you check the man page? strlen(3) returns size_t. Use %zu to print it.

As mentioned in the comments below, clang is sometimes helpful with finding better error messages. clang's warning for exactly this case is pretty great, in fact:

example.c:6:14: warning: format specifies type 'unsigned int' but the argument
      has type 'size_t' (aka 'unsigned long') [-Wformat]
    printf("%u\n", strlen("abcde"));
            ~^     ~~~~~~~~~~~~~~~
            %zu
1 warning generated.