Mostly I can't get the font size to work neither on my user profile nor my pet profile.
BUYING PIXEL ART FOR MY PROFILE
I want to make text with both the link and a big size since the default size is too small, but it looks like pairing it with a link breaks the size BBCode.
Are you talking on the forums or in your pet coding? Those are two different things. Anything off the forums is CSS. Your nesting is correct in the code shown to work on the forums.

So I'm stuck again and I can't find what's wrong.
<style type="text/css">
.font15 {
font-size: 15px;
color: ;}
</style>
BUYING PIXEL ART FOR MY PROFILE
It was working before I added the color in. What happened?
Are you adding that into your sig? That won't. Sigs are bbcode only.

Nope, on my user profile.
link code is actually different than the regular text code! so on your profile instead of doing .font15, please try this instead:
this time, write out your link normally and it should work!
Now all my profile links are big and still no color change 🤔
<style>
a{font-size: 15px;
color: ;}
</style>

Trying to learn how to post gif. Please forgive.
[center]
[center]
If you wanna change links for a whole class use: .class a {}
You can also do span .class span a {}
Spans are like divs expect they stand for "any text" rather than "a box"
Also try this:
Color:white!important;
The important is needed sometimes
Ping me if this didnt help, ill take a closer look
you can make a custom class with the styling you want, and apply it just to that text. something like this inside the style tags:
.biglink a, .biglink a:link, .biglink a:active, .biglink a:visited {
font-size: 15px!important;
color: !important}
.biglink a:hover {color: !important}
and then outside the style tags, you would put: