UILabelのsizeToFit

// ラベルの作成
UILabel* label = [[UILabel alloc] initWithFrame:CGRectMake(0,0,200,100)];
label.text = @"ABCDEFGHIJKLMNOPOQRSTUVWXYZ";
label.textAlignment = NSTextAlignmentLeft;
label.font = [UIFont systemFontOfSize:16];
label.backgroundColor = [UIColor grayColor];
label.textColor = [UIColor whiteColor];
label.numberOfLines = 2;
[self.view addSubview:label];

// ラベルの作成
UILabel* label2 = [[UILabel alloc] initWithFrame:CGRectMake(0,100,200,100)];
label2.text = @"ABCDEFGHIJKLMNOPOQRSTUVWXYZ";
label2.textAlignment = NSTextAlignmentLeft;
label2.font = [UIFont systemFontOfSize:16];
label2.backgroundColor = [UIColor blueColor];
label2.textColor = [UIColor whiteColor];
label2.numberOfLines = 2;
// --- sizeToFitする ---
[label2 sizeToFit];
[self.view addSubview:label2];

実行結果