Good question!
You don't need to set max-width. You just need to set a width for smaller devices using media queries of CSS3.
Just open the tooltip.css file, and add the following style:
@media (max-width: 900px) {
.mcTooltipWrapper, .mcTooltipInner {
width:200px!important;
}
}
Note: you can surely change the 900px in above code to whatever you prefer such as 800px, 1100px, etc.