int someStrLen;
char Val[10];

if (LOWORD(wParam) == IDC_Byte)
{
  GetDlgItemText(hDlg, IDC_Entry2, Val, 10);
  if (Val != NULL)
  {
         someStrLen = strlen(Val);
         SetDlgItemText(hDlg, IDC_EXIT2, // Must have someStrLen value);
  }        
  else
  {
        SetDlgItemText(hDlg, IDC_EXIT2, "Error");
  }
}

int someStrLen;
char Val[10];

if (LOWORD(wParam) == IDC_Byte)
{
 GetDlgItemText(hDlg, IDC_Entry2, Val, 10);
 if (Val != NULL)
 {
        someStrLen = strlen(Val);
        char *int_to_str = (char*)malloc(20); //should be enough to hold any assignable char buffer on 32 bit systems
        _itoa(someStrLen, int_to_str, 10);//C std call to convert int to char, declared in stdlib.h

        SetDlgItemText(hDlg, IDC_EXIT2, int_to_str);
        free(int_to_str); /*don't forget to free the buffer after use. Consider using a permanent buffer for the scope or a char[] within local scope so that stack releases it on its own*/
 }        
 else
 {
       SetDlgItemText(hDlg, IDC_EXIT2, "Error");
 }
}

int someStrLen;
char Val[10];

if (LOWORD(wParam) == IDC_Byte)
{
 GetDlgItemText(hDlg, IDC_Entry2, Val, 10);
 if (someStr != NULL)
 {
        someStrLen = strlen(someStr);
        SetDlgItemText(hDlg, IDC_EXIT2, itoa(someStrLen, Val, 10));
 }        
 else
 {
       SetDlgItemText(hDlg, IDC_EXIT2, "Error");
 }
}