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.
