想要设置UIButton的Image大小最简单的一个方法
UIButton *button = [[UIButton alloc]initWithFrame:CGRectMake(15, 345, SCREEN_Width-30, 50)];
[button setImage:[UIImage imageNamed:@"scan"] forState:UIControlStateNormal];
[button setTitle:@"扫码添加" forState:UIControlStateNormal];
[button setTitleColor:mainColor forState:UIControlStateNormal];
[button setImageEdgeInsets:UIEdgeInsetsMake(15, 0, 15, 0)];
button.imageView.contentMode = UIViewContentModeScaleAspectFit;
[self.view addSubview:button];
设置UIButton 的 contentMode 可以让图片等比例缩放，设置 ImageEdgeInsets 可以缩小图片，必须两个同时设置才能起效果，让一张在button里面过大的图片等比例缩小到你想要的大小.
样图
