Jay wrote:
Is there a guide anywhere that would determine proper oar size (i.e.
length) for an open rowboat that would take into account the
measurements of said rowboat?

Put the blade end of your 7' oar against the inside transom and lay
the oar across the seats lengthways in the boat as close to each side
as you can get them. If the curve of the sides leaves too much wasted
space between the the oars and the sides, your oar is too long. Cut it
off LoL!
Seriously, for back up, 7' should be fine. I have 7' oars and I have
no trouble at all rowing my 14' OR my 16' skiff. If they feel too
short, just shove 'em outboard a little and keep on rowing.
But try your 7 footers before you buy anything longer. And think about
how you're going to store anything longer too. it. If you honestly
think 7' won't work, buy something longer but get something you can
take apart for storage. And remember, you may never use them anyway.

Rick