jQuery Font Switcher
I felt that this code would be good as a jQuery plugin, so I went ahead and altered your code until I was satisfied. I left in some comments that are applicable to your code, but I would encourage someone else (and may do it myself still, I don't know yet) to give an actual review.

(function ($, console) {
    'use strict';

    var defaultOptions = {
        //markup is what will be used to create the font sizer widget; you can do this a million better ways, but I felt this was easy to understand.
        //I'd consider enhancing it to take a jQuery object or a function that returns a string or a jQuery object, but all of that would make this more complex.
        //At the very least, this way makes for progressive enhancement as opposed to breakage when JS is disabled.
        markup: 'Font size: &lt;a href="#smaller" data-make="smaller"&gt;Smaller&lt;/a&gt; | &lt;a href="#bigger" data-make="bigger"&gt;Bigger&lt;/a&gt; Current Font Size:&lt;span data-id="current_size"&gt;&lt;/span&gt;',
        markupClickFilter: 'a', //if you change the markup of this widget to something where you use elements other than A tags for buttons, you may wish to change this
        affects: null, //required: this is a jQuery object representing the elements that this widget affects
        size: null, //override for initial size of affected elements
        increment: 4
    };
    $.fn.fontSizer = function (options) {
        //initialize / preconditions
        var opts = $.extend({}, defaultOptions, options),
            $currentSize, //$ for element in markup if it exists
            sizeChange = function () {}; //noop to avoid null check when using this, meh...
        if (!opts.affects || !opts.affects.length) {
            //should always UI test in a modern browser that provides console
            console.error('failed to initialize fontSizer: must provide affects property');
            return this; //jQuery plugins are expected to follow this convention
        }

        //widget setup
        this.html(opts.markup); //again, this setup functionality could be done better in many ways
        $currentSize = this.find('[data-id=current_size]'); //for example to not use magic strings like this
        if ($currentSize.length) {
            sizeChange = function () { //this could also be a trigger
                $currentSize.html(opts.affects.css('font-size'));
            };
        }

        //set initial size and update $currentSize
        if (opts.size !== null) {
            opts.affects.css({
                'font-size': opts.size
            });
        }
        sizeChange();

        return this.on('click', opts.markupClickFilter, function (e) {
            var size = parseFloat(opts.affects.css('font-size')), //parseFloat is more correct than parseInt
                make = $(this).data('make'),
                fixed = parseFloat(make); //minor enhancement: allow fixed sizing

            if (make === 'smaller') { //preference: always use ===
                size -= opts.increment; //a new variable was unnecessary, if you feel you must, you should at least declare it at the top of the function
            } else if (make === 'bigger') {
                size += opts.increment;
            } else if (fixed !== 0) {
                size = make; //using make instead of fixed here allows things like make === '1.5em'
            }

            //update size (and set $currentSize)
            opts.affects.css({
                'font-size': size
            });
            sizeChange();

            //this prevents the annoying hashchange and extra history step that isn't doing anything useful
            e.preventDefault();
        });
    };
}(jQuery, console || {error: function () {}}));


Usage:

(function ($) {
    'use strict';
    $(function () {
        $('#sizer').fontSizer({
            affects: $('p.basic')
        });
        $('#sizer2').fontSizer({
            affects: $('p.custom'),
            markup: 'Font size: &lt;a href="#small" data-make="12px"&gt;Small&lt;/a&gt; | &lt;a href="#lg" data-make="24px"&gt;Large&lt;/a&gt; | &lt;a href="#xl" data-make="36px"&gt;XL&lt;/a&gt; Current Font Size:&lt;span data-id="current_size"&gt;&lt;/span&gt;'
        });
    });
}(jQuery));


Html for usage example:

&lt;div id="sizer"&gt;Enable Javascript for a font resizer&lt;/div&gt;

&lt;p class='basic'&gt;some text&lt;/p&gt;
&lt;p class='basic'&gt;some text&lt;/p&gt;
&lt;p class='basic'&gt;some text&lt;/p&gt;

&lt;div id="sizer2"&gt;Enable Javascript for a font resizer&lt;/div&gt;

&lt;p class='custom'&gt;some text&lt;/p&gt;
&lt;p class='custom'&gt;some text&lt;/p&gt;
&lt;p class='custom'&gt;some text&lt;/p&gt;


JsFiddle