How to change width, font size and other basic edits for the posts?

Are you talking about the front or back end of your site?

Download the plugin Jetpack and use its Custom CSS functionality to hold your CSS within an area of the dashboard.

O! I don’t know if it is going to help but I’ve fallen in love with introduction to the CSS Beginner Tutorial:
“Like the HTML Beginner Tutorial, the CSS Beginner Tutorial assumes that you know as much about CSS as you do about the cumulative effects of sea squirt discharge on the brain chemistry of Germanic ammonites.”
That’s exactly as it is.

esmi wrote:
“Are you talking about the front or back end of your site?”

Thanks for your question. I’m talking about the font and other page formatting capacities of the “Comment”menu and what happens when my anticipated visitor/member will try to make a post. I am talking about formats of such posts: text box width, font size, hyperlinks/references, picture/video embed options etc. ??

Andrew Nevins wrote:
“Download the plugin Jetpack and use its Custom CSS functionality…”

No. It doesn’t work enough too. It even doesn’t tell what part of CSS it is going to change and it doesn’t change what you need to be changed but something else.

I’m talking about the font and other page formatting capacities of the “Comment”menu

There isn’t a formatting menu for comments on the front end of WordPress by default, so I can only assume that you;re using a comment plugin. Can you confirm this?

I meant “Edit” menu. You may post your post in the bad designed half monitor’s screen size “comment form” without any edit capacities and return back to your post later in the “Edit” menu to make your post little-bit better if the “edit” menu works. The edit menu form doesn’t have any text size/font etc. format buttons and “?html?” format which is called “Text” doesn’t work for html tags (I suspect some conflict between html tags and general CSS assignments).
I tried to install plug in of Mr. Planc on external edit menu but it doesn’t work constantly creating 2 errors for me. I was trying suggestion here given by Mr. Andrew Nevins on the Jetpack correcting CSS but it changes all website to the same format if it is changing something at all.

No. It doesn’t work enough too. It even doesn’t tell what part of CSS it is going to change and it doesn’t change what you need to be changed but something else.

You’ll need to learn some basic CSS in order to make changes that work the way you want. If you use specific “selectors” the CSS will target only selected elements.

https://www.w3schools.com/css/

Also, try using Firebug to help you work with CSS – it shows you the CSS code that is affecting an element on the page – then you can copy that CSS code to your custom section and make the desired changes there.

You’ll need to learn some basic CSS in order to make changes that work the way you want. If you use specific “selectors” the CSS will target only selected elements.

https://www.w3schools.com/css/

Also, try using Firebug …

Thank you for your good advices and URLs to make advices true. I’ll do my best learning and it will definitely help even if I’ll need to give up with WordPress.