Clive, in looking at your wonderful painting of your son, I believe your approach to portraiture should be more adventurous and modern. Your natural style does not seem to fit in the 'classical' mode.
Check out this site on Fairfield Porter,
http://www.artchive.com/artchive/P/porter.html#images
He is one of my favorite painters. His use of color was phenomenal. He used broad areas of bright color to define his space. Though not the most skillful of draughtsmen he nevertheless created portraits of powerful simplicity. Van Gogh was not that skillful either, but I would rather have one of his iris paintings than a roomful of Bouguereaus.
The painting "The Mirror" of his daughter, reminds me of your son's picture.
As to the size of your painting, I go by head size. I like to use an 8" head and base the size on that. You could probably get away with a 7" head on this one, but I wouldn't go any smaller. Work out your exact size on tracing paper and get some stretchers made.You can get a smaller drawing blown up at a place that does work for architects. Do not try to fit this picture into readymade stretchers. Chris Polsen of Twin Brooks Pro Stretchers will make you a set to your precise measurements. He's fast, reasonable and they are beautiful. His phone number is 1 800 856-1567.
I hope this helps. Sincerely,