Custom navbar sizing from iphone 5 to iphone 6

I have a custom tab bar that was optimized for the iphone 5.

UINavigationBar *myBar = [[UINavigationBar alloc] initWithFrame:CGRectMake(0, 0, 320, 50)];
[self.view addSubview:myBar];

When it on the iphone 6, part of it is cut off of course because it only goes to 320 pixels.

How do i fix this? Is there a way to check which iphone its being ran on, and then run the pixel specified code? I plan on putting a background image on this navbar later so it must be centered.