From a racing standpoint, I don't need a large reservoir. My question is then: how large a one is really needed to accomodate the expansion and contraction of the coolant during normal operation.

I don't need it for "just in case" because I always look at fluids at the end of every road race so water losses are immediately corrected.

 

NHRA - for example - requires that you have an overflow tank, and that it be at least 1 pint.

Try a "search". Someone asked this exact question in the last few days, on this forum, and I posted a suggestion on how to figure the required volume.