How do I change the font size and type only for message ?

[TRABZONLUKAPLAN]

When a member starts to write a message, I want the font size is 3 and the type of font should be "arrow narrow". How do I make this default ? But I only want to change this for messages not for any other sections.

[Gio~Logist]

All style options -> Input Fields -> (CSS Selector: textarea, .bginput_)

That will make the text appear bigger by default.