Text sizes, what's readable?

I've found out that some readers have trouble reading text when I set it to be one step smaller than Normal. (I do this sometimes to de-emphasize a sentence.)
Which sizes below are easy to read for you? (Well, the text anyway, not the poetry.)

Normal:

It is an ancient Mariner,
And he stoppeth one of three.
"By thy long grey beard and glittering eye,
Now wherefore stopp'st thou me?

Minus One:

It is an ancient Mariner,
And he stoppeth one of three.
"By thy long grey beard and glittering eye,
Now wherefore stopp'st thou me?

Normal, italics:

It is an ancient Mariner,
And he stoppeth one of three.
"By thy long grey beard and glittering eye,
Now wherefore stopp'st thou me?

I haven't used it before, but maybe some can even read Minus Two?:

It is an ancient Mariner,
And he stoppeth one of three.
"By thy long grey beard and glittering eye,
Now wherefore stopp'st thou me?

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I have always found that the default text size in browsers and on web sites are often too small for me to read comfortably, so I have always set my browsers to display it bigger by default, and to have a bigger minimum size. I don't know what percentage of web surfers are aware they can do this.
I can read pretty small text, but I find that a nice big font puts a lot less strain on the eyes, which is nice when you use screens alda time.