First let me thank you for great work. Thanks to you we have reliable editor with everything that articles authors could dream of :-)

Thank you!

My question is if there is a way how to change font size of the editor - I mean relative change of the content only. I have a visual impairment so representation of the contents becomes too small to me periodically. I would appreciate to have feature to "magnify" a little actual content so writing would be more comfortable to me. IE not to change font sizes of the article itself, but only for me in the editor frame (without magnifying whole page displayed in browser, which would distort toolbars etc), uhm I hope I'm clear. Is it possible please?

The font size of the editor content is controlled by the site template, but it is possible to make changes that only affect the editor content.The easiest way is to add some css to your template's user.css or custom.css file that contains the font-size increase, scoped to the editor's body class, eg:

.mceContentBody {
    font-size: 16px;
    line-height: 1.5em;
}
/* these sometimes have indiviual styling in templates so we need to set these too */
.mceContentBody p,
.mceContentBody td, {
    font-size: 16px;
    line-height: 1.5em;
}

Which template are you using?