In k-meleon 0.65 the file macros.cfg contains the macro 'fontsize'.

IMHO shold this file containing the macro minfontsize_up and minfontsize_down too:

minfontsize_up {
menu = "Font-minimum-size up";
$newsize = getpref(INT, "font.minimum-size.x-western") + 1;
setpref(INT, "font.minimum-size.x-western", $newsize);
statusbar("font.minimum-size set to " . $newsize);
}

minfontsize_down {
menu = "Font-minimum-size down";
$newsize = getpref(INT, "font.minimum-size.x-western") - 1;
setpref(INT, "font.minimum-size.x-western", $newsize);
statusbar("font.minimum-size set to " . $newsize);
}

And in accel.cfg:
ALT VK_PLUS = macros(minfontsize_up)
ALT VK_MINUS = macros(minfontsize_down)

And in the file prefs.js
user_pref("font.minimum-size.x-western", 12);

there's a discussion starting on the dev list right now about what to include in the config files (macros, accel, toolbars, etc...) by default... you may want to join in, since i'm not sure how often the devs read these forums anymore, what with all the entropy...