This is an extremely easy "fix"
I do it practically every time I view a lesson
...adjusting the size of font in the main window (or adjusting the size of the Tab window itself)
for various reasons; without even really thinking about.

whilst the size of the text in the pop-up tab window will adjust via keyboard shortcuts like (holding "command") and tapping "+"
What that's doing is increasing/decreasing the image size (the tab is an image; in the pop-up window) 
...and it will simultaneously increase/decrease the size of the text in the video-window as well.
(at least with google-Chrome)

So what I do is adjust the size of the pop-up tab window itself; by "clicking and holding" on the edge of the window and "dragging" using the cursor/pointer.

However
It might be tricky to learn by reading some text about it;
especially if computers are still quite alien to you;
and guessing from the question... they are.

One can web-search for a video explaining practically ANYTHING
The trick is knowing what things are called.

imo the key problem here is you are way behind when it comes to computers.
I'm behind; but not way behind.

Most adults stop being tech savvy past public school
because there's not a system forcing them to buckle down and stay up to pace with things like technology; and they have no strong desire to.

So

IF you don't already know this stuff
THEN you'd be well-served to take an entry level computer class so you'll know what things are called; and then can web search for vids in regards to any question you might have.

many libraries have them for in person instruction.

otherwise there's various online vids on youtube

your local librarian will be able to demo this as well as help you find a good course 

For best results find the youngest librarian you can