点击按钮修改文本尺寸

创建UILabel

self.lbl=[[UILabel alloc] initWithFrame:CGRectMake(135, 290,200,35)];
    self.lbl.backgroundColor=[UIColor clearColor];
    NSString *str=[NSString stringWithFormat:@"%.f",[self.slider value]];
    [self.lbl setFont:[UIFont fontWithName:@"Arial-BoldMT" size:16]];
    self.lbl.text=str;
    [self.View addSubview:self.lbl];

点击UIButton时修改UILabel文本尺寸

-(IBAction)ButtonPressed:(id)sender
{
    [self.lbl setFont:[UIFont fontWithName:@"Arial-BoldMT" size:25]];
}

应该可以实现你想要的效果 :)