
Quoting
Otis B Driftwood
For the type restriction on the sign you violated, several of the R30 signs up to and including several of the R38 sign variety would work for a No Parking -Certain Times sign. But you're not going to find a specific size sign or a particular size font. While you might find a 18" X 30' sing, or a 12" X 18", the regulatory authority can choose to blow the sign up into twice those sizes if they so choose.
From Section 2E.14 Size and Style of Letters and Signs on page 399 of the 2012 Ca MUTCD, it says:
Sign size is determined primarily in terms of the length of the message and the size of the lettering necessary for proper legibility.
And so the 1.5 inch size you're attempting to make into a "minimum" isn't so. It simply is the associated height of the letters that would allow that long of a message to fit on that size sign.
In fact here is another similar sign and you can see that the small letters there are much shorter than 1.5 inches:
http://www.dot.ca.gov/hq/traffops/si...ecs/R38(S).pdf

Quoting
Otis B Driftwood
How do I establish that the type on the sign is too small. Do I need to or can I just assert that it is?
The reasonable man rule will prevail here... If a reasonable man were to look at that sign, would he be able to read it and understand it?
No offense intended but the answer is "yes". For someone standing that close to it who is not looking at it through an unfocused camera, it is neither illegible nor confusing!