In your CSS file, there are some lines:
#sidebar ul li {
list-style:none;
line-height:1.6em;
font-size: 1.1em;
}
Change the 1.1em to change the size of the text on the right sidebar. [If you recently added those lists you may have changed things so that this rule is the one that takes precedence.]
In your left sidebar (at this point anyway) you’ve got explicit font tags to set the size. If you changed your table in that sidebar to a list you could use the CSS rule above to apply to that sidebar too, and have an easier time keeping things consistent.
CSS can be your friend — but it takes awhile to figure it all out.
The validators at https://validator.w3.org/ and https://jigsaw.w3.org/css-validator/ can sometimes help figure out where things are going wrong. But the primary tool I used to look at your site was the Firefox Web Developer extension that offers tools to view and edit CSS markup.