The label on the jacket appears to show a "38 Short" sizing.
JFK was at least 6" tall so, it seems he would have worn a jacket of a longer length.
A quick Google search shows JFK wore a "32 Regular" suit size. The JFK Presidential Library site says that JFK wore shirts with a 34" sleeve length - this would be more in line with a Regular suit sizing. Also the JFK site says his hat size was 7 3/8. I could not see the sizing label for the hat on the auction.
I think that the claim that these are JFK's uniform jacket and hat are suspect.