How to make width uniform across font sizes?

Hello,

I was hoping someone might have an idea for what type of code I can put in my CSS in order to have my horizontal menus have equal widths when they contain two separate font sizes.

Currently, my nice menu #1 has a font size of 12px, and then the one below it, nice menu #2, is at 11px. The issue is that nice menu #1 auto expands for the larger font size, which is unnecessary in my case (see screenshot).

In my theme's CSS, I tried putting in a code like this:
#nice-menu-1{
width: 13em;
}

...which is a bit shorter than the width I'd set in the nice menu default CSS, but that didn't work. I also tried putting "width: fixed" and "width: absolute" but that didn't change anything.

Thanks for any input!