Change Font for Custom Menu Widget (and only in this widget)

Sirs: I want to add a custom menu in a page, so I 1) created a custom 2) creates custom widget area, 3) put my menu there and, 4) add the custom widget to my page. Fine.

But the font is too small. I fiollowed your advice in this post (https://kriesi.at/support/topic/increase-font-size-on-custom-menu-widget/) to increase the font, with this code:

.widget ul li a {
font-size: 12px;
}

The problem in that this code changes the font for all widgets in my side bars.

Which will be the code to change only this widget?