Q. Biggest Problem I face is (a) The Fonts size in compose windows are shown as small or Large font size, why don't you give number size like font size 5/6/7..
Biggest Problem I face is (a) The Fonts size in compose windows are shown as small or Large font size, why don't Thunderbird (developers) give number size like font size 5/6/7.. as giving number to font make you sure the size is correct for the font.
Alla svar (2)
Why do people expect to set font sizes in email? You set your own devices and programs to suit you. You don't want the sender or author to override your preferences. Nor should you try to control how others see your messages to them.
Set Thunderbird to compose using "medium" or "normal". Then the size for the recipient will be whatever they consider to be normal.
For yourself, go to Tools|Options|Display|Formatting->Advanced and set the font and size to what suits you. This setting applies to your incoming messages too, so you will see your mail in sizes and typefaces you choose, not what the sender felt like using.
Email is not wysiwyg. You have no absolute control over how your correspondents see your messages to them.
Sometimes you NEED what you type to be wysiwyg. Otherwise your spacing into columns, or use of "ASCII Art" to make a crude diagram, gets effed up. I do a lot of those crude diagrams.