If you are not using a child theme, the changes can be done with just using custom CSS via a plugin like Simple Custom CSS. For the site title, this has a few font sizes based on device viewing from desktop to phones…example, the theme has this for viewing windows that are larger than 992px (desktops)
@media (min-width: 992px) {
.site-title {
font-size: 4.75rem;
}
}
For smaller windows like tablets:
@media (min-width: 768px) {
.site-title {
font-size: 3rem;
}
}
Then for phones, this is the default theme styling for the site title size and font…and a few other things:
.site-title {
padding: 10px 0;
font-family: "Old Standard TT",Georgia,Serif;
font-size: 2.5rem;
line-height: 1;
text-align: center;
}
You can copy the code snippets above and add them to your own custom CSS and then just change the font-sizes to your own preference. You may notice that I use rem for font size, but you can use em % or even px…but best to use rem. To know what an rem unit is in pixels, you can use a reference table from https://www.pxtoem.com for that. It converts px to em (rem) units.
For the text widget, which area of the page were you wanting to adjust this font size for? The left and right sidebars, the footer or bottom area? Or all text widgets?