当时想了很久的一个问题解决办法：
对于直接在barbuttonItem上加的uibutton是不能够改变尺寸大小：
UIButton *backBtn = [UIButton buttonWithType:UIButtonTypeCustom];
[backBtn setImage:[UIImage imageNamed:@"btn_back1"] forState:UIControlStateNormal];
[backBtn addTarget:self action:@selector(backBtnClicked:) forControlEvents:UIControlEventTouchUpInside];
backBtn.frame = CGRectMake(0, 0, 40, 40); //改不了尺寸，会被拉伸
[backBtn setTitleColor:[UIColor blackColor] forState:UIControlStateNormal];
UIBarButtonItem *barButtonItem = [[UIBarButtonItem alloc]initWithCustomView:backBtn];
self.navigationItem.leftBarButtonItem = barButtonItem ;
需将button加在一个UIView上，再将UIView加在barbuttonItem上，只需改变UIView的size即可：
UIView *contentView = [[UIView alloc] initWithFrame:CGRectMake(0, 0, 44, 44)];
UIButton *backBtn= [[UIButton alloc] initWithFrame:contentView.bounds];
[backBtnsetImage:image forState:UIControlStateNormal];
[contentView addSubview:backBtn];
UIBarButtonItem *barButtonItem = [[UIBarButtonItem alloc] contentView];
self.navigationItem.leftBarButtonItem = barButtonItem ;
