Thanks for your help and suggestions - but I'm afraid I'm still in the same situation.
I know how to increase and decrease the font size on Thunderbird [<View> then <Text Size> then <Increase> or Decrease> provides five different text sizes in total] but, although text size is fine when on screen, it comes out in very small print when hard copy is taken. If I try to print a web page though the text size in print comes out at the same size as on screen.
I'll keep thinking and trying!!