UINavigationBar

//.h file

#import <UIKit/UIKit.h>

@interface UINavigationBar (customNav)
@property (nonatomic) NSValue *customSize;

@end

//.m file

#import <objc/runtime.h>
NSString * const kCustomSizeKey = @"kCustomSizeKey";


@implementation UINavigationBar (customNav)
- (CGSize)sizeThatFits:(CGSize)size {
    CGSize newSize;
    if (self.customSize) {
        newSize = [self.customSize CGSizeValue];
        return newSize;
    }
    else {
    	return size;
    }
}

- (void)setCustomSize:(NSValue *)customSize {
	objc_setAssociatedObject(self, (__bridge const void *)kCustomSizeKey, customSize, OBJC_ASSOCIATION_COPY);
}

- (NSValue *)customSize
{
	return objc_getAssociatedObject(self, (__bridge const void *)kCustomSizeKey);
}
@end