I use helvetica 6, which is just a tad smaller than I'd like, but 7 is just huge.
There is no technical reason why the user can't specify the exact size he wants, except to keep the UI simple. It's just simpler to present 8 choices, rather than to have a slider or a +/- numeric input. Nice and easy, unless it's not what the user wants.
Which is why you need to have some other way for those willing to go the extra yard to get what they want. Like an advanced settings screen. Or an editable config file. As it is, you need someone to hack the code, so it's either real easy or real hard.