Not that this is what you are gonna want to hear, but the legal requirement is stated as 14 scan lines. that is readable on a scope. Font sizes may vary due to the structure of the fonts. So a font size is not always the same therefore saying a 14 line font is not necessarily 14 scan line. Ya gotta have a scope and count the scan lines. that is the only safe way to do it.
I think it depend on your particular place and application.
We are a production company in Alabama and do a fair little amount of political commercials, as well as doctors, dentists, car dealers with lease offers... all frequently needing disclaimers.
At least in our area, for political disclaimers ("Paid political advertisment by committe to elect...") the requirement is 20 scan lines, up for a minimum of four seconds.
There are further particulars, for example for a local election (mayor, etc.) the disclaimer can come anywhere (even in the middle of a spot). But for a statewide election (governor, etc.) it MUST be either the first :04 or last :04 in the spot.
For other types of uses (such as advertising for attorneys or dentists: "No representation is made that the services....") there is a completely different set of rules.
All that's a long way to say, I don't think there IS one standard... it just depends on what kind of "legal disclaimer" you are talking about.