Duane, I took a 980 Kb PDF and saved it as a PNG. The PNG is viewable here: http://tinyurl.com/cfgy63p . The PNG is 376 Kb.
Having said this, I am confused about "appropriate file size". Are we still concerned about shrinking map sizes? I do not know how much better posted maps may appear if we use file sizes of 1 Mb or larger. But it cannot hurt.
I also contend that a better scanning procedure, regardless of file size, can help. I had to research some USATF online maps over the weekend. Some of them are scanned with the orientation rotated 90%. Fine for printing, but an annoyance to view on screen or to "PDF" them to send to a client.