If you mean on your computer it all depends on your operating system,(in Windows 7 it's under Fonts after your

select Control panel).

Via the discussion boards it's via the Font size above the reply box.

I use Turbo lister for the listings and you can change the size in Description builder.

There is also an option to change the font size via the Description builder when you edit the listing,(I presume it

works the same way when you first do your listing via eBay itself),

Or if your are using Windows 7 you can just hold down the CTRL key on the keyboard while you operate the scroll wheel.

This will increase/decrease the zoom so you display the fonts and everything on the screen bigger/smaller.