How to increase writing font size in thunderbird?

I decide to write this article because I was facing problem with writing messages on thunderbird platform, and then read the written text to make a final confirmation of message and send them to recipients. 

The size of the fonts of written text was so small that I had to make a pressure on my eyes to focus on texts for reading. Definitely this was not good for eye health. 

What you have to do if you are in a situation as I was. 

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It needs only to make few changes in settings of thunderbird to enlarge the size of the fonts of compose message. 

How I have adjusted the font size?  The images which I have taken are in Spanish but the steps are same in English thunderbird.

Step 1: Open you thunderbird email platform.

Step 2: Click on <Tool> as you see in below photo.  

Step 3: Click on <Account Configuration>. 

Step 4: Click on <Composition and Addressing>. 

Step 5: Click on <Global Composing Preference>. 

Step 6; Click on <General>. By default it will select Composition but you have to change to General. 

Step 7; Click on <Language and Preference>. Here you can change writing style and size of the font. 

Step 8: Click on <Advanced> to find more options. 

Step 9: Close the window of settings of thunderbird. 

Step 10: Compose a message and you will find the change in font size.