Just as an example:
I choose to add a selectbox-option. Because when you add a slider you would have to add a background image to the gantry.css for the "default-medium-small" option like "high-low-med". Not that hard to do but a selectbox is easier.

1. add this to your templateDetails.xml:

<param name="my-menusize" type="selectbox" default="default" label="MENUSIZE" description="" setbyurl="true" setinsession="true" setbysession="true" setincookie="true" setbycookie="true" isbodyclass="true">
<option value="default">Size default</option>
<option value="medium">Size medium</option>
<option value="small">Size small</option>
</param>

you can add it right after the "backgroundlevel"-Parameter

2. add this to your template.css (or demo.css or a custom css-file)

/*Menusize feature*/
.my-menusize-small .menutop li.root > a.item
{font-size: 14px;padding: 0;height: inherit;
line-height: 30px;text-transform: capitalize;}
.my-menusize-small div.rt-menubar
{margin-top: 80px;}
.my-menusize-small .menutop li.parent.root a.item span .daddyicon,
.my-menusize-small .menutop li.root a.daddy.item .daddyicon
{top: 12px;}

.my-menusize-medium .menutop li.root > a.item
{font-size: 16px;padding: 0;height: inherit;
line-height: 60px;text-transform: capitalize;}
.my-menusize-medium div.rt-menubar
{margin-top: 50px;}
.my-menusize-medium .menutop li.parent.root a.item span .daddyicon,
.my-menusize-medium .menutop li.root a.daddy.item .daddyicon
{top: 27px;}

3. clear your caches.
This gives you a nice option for the menu-size in the template-settings