If wartime you need a 48 star flag:
And not a 50 star Modern National Ensign:
The pattern of stars in the canton (the blue bit) will "show" at even small scale. The Union Jack must match, too.
As some trivia, when I was born, this was the US flag:
The 49 star flag basically only flew for a year, before being replaces 4 July 1960 after the admission of Hawai'i as th e50th State.
Submarines were one of the few ships not (necessarily) required to fly their Call Sign letter flags while under way in Harbor or Approach Pilot jurisdiction. This reflects two things, one that the signal flags would be tiny in #7 (sub & small craft) sizing, and that submarines are not generally considered vessels that Pilots can manover.