I've just compiled and run it on my UNIX box without problem. You could try changing cin>>data; to cin>>data;. Other than that (and an extra semi-colon on you int size = 0; line which shouldn't cause this) I can't see anything wrong.
If that doesn't fix it, post your compiler details, someone may be able to be of more help than me.
int size = 0; //removed extra ; (would not cause an error)
cout << "please input the word\n";
cin >> data; //changed from: cin >> data;
size = strlen(data);
cout << "the number of letters is " << size;
EDIT: Beaten by bnbertha, but on my Windows box I needed to change cin >> data;