How to change the uitextview font size

Have you bind textView with xib or allocated textView?

If yes,

You can use following code to check whether this works

[textView setFont:[UIFont boldSystemFontOfSize:15]];

or

 [textView setFont:[UIFont systemFontOfSize:15]];

Check, if "Selectable" is checked in the xib/Storyboard attribute inspector. If it's not selectable, the size wont be changed