The reason that the code you're posting isn't working is probably because Arial isn't present in iOS. You can change the properties of a
titleLabel just like any other instance of
UILabel. Read about that here.
So, you can change the font size with something like:
button.titleLabel.font = [UIFont systemFontOfSize:14.0];
You can choose a font by name, but not all fonts are available on iOS. Here's a list of ones that are.