For this you can customize the title label of UINavigationBar. You can set its minimumFontSize property so make the text adjustable.
OR
For iOS 6 you can use below code, so that you can provide a custom label:
UILabel *label = [[UILabel alloc] initWithFrame:CGRectMake(0, 0, 120, 30)];
label.textAlignment = UITextAlignmentCenter;
[label setFont:[UIFont boldSystemFontOfSize:16.0]];
[label setBackgroundColor:[UIColor clearColor]];
[label setTextColor:[UIColor whiteColor]];
[label setText:text];
[self.navigationController.navigationBar.topItem setTitleView:label];