ios7及其以前都可以用
sizeWithFont:方法，到iOS8开始废弃
sizeWithFont方法，改用boundingRectWithSize:方法，具体请参考如下方法
//获取lable高度
- (CGRect)getSize:(NSString *)str{
CGRect labelSize = [str boundingRectWithSize:CGSizeMake([UIScreen mainScreen].bounds.size.width-20, 1000) options:NSStringDrawingUsesLineFragmentOrigin attributes:[NSDictionary dictionaryWithObjectsAndKeys:[UIFont systemFontOfSize:13],NSFontAttributeName, nil] context:nil];
return labelSize;
}
