iOS之改变BarButtonItem中显示的字体大小

UIButton * button_back = [[UIButton alloc] initWithFrame:CGRectMake(0, 0, 44.0f, 44.0f)];
[button_back setCenter:CGPointMake(self.view.frame.size.width/2, self.view.frame.size.height/2)];
[button_back.layer setCornerRadius:4.0f];
[button_back.layer setBorderColor:[UIColor blackColor].CGColor];
[button_back.layer setBorderWidth:1.0f];
[button_back setTitle:@"back" forState:UIControlStateNormal];
[button_back setTitleColor:[UIColor grayColor] forState:UIControlStateNormal];
[button_back.titleLabel setTextAlignment:NSTextAlignmentCenter];
[button_back addTarget:self action:@selector(backButtonClick:) forControlEvents:UIControlEventTouchUpInside];

UIBarButtonItem *backButton = [[UIBarButtonItem alloc] initWithCustomView:button_back];
[backButton setStyle:UIBarButtonItemStyleDone];
[self.navigationItem setLeftBarButtonItem:backButton];