Answered: How do I change the font size in comments ?

In most themes, for example SnowFlat, which is the default Q2A theme, you need to open your theme folder > and open the file qa-styles.css .

And paste the following code at the very bottom of the file:

.qa-c-item-content {
    font-size: 14px;
}

Change the pixels value for a bigger number to increase the font. 

If you want to increase the font size for mobile devices only, you would need to wrap the code above in a media query, like the example below:

@media (max-width:480px) {
    .qa-c-item-content {
        font-size: 14px;
    }
}

You might need to adjust the media query max-width value depending on your needs.
There's an article here you can read, about Media Queries for Standard Devices.