Annual Appeal: Please make a donation (tax deductible in USA) to keep the OEIS running. Over 5000 articles have referenced us, often saying "we discovered this result with the help of the OEIS".
a(5)=6 since A077722(5)=32833, which has an octal form of 100101 (6 digits).
MAPLE
s := 0:for n from 1 to 7000 do b := convert(n, base, 2): q := sum(b[i]*8^(i-1), i=1..nops(b)): if(isprime(q)) then s := s+1:a[s] := nops(b):fi: od:seq(a[k], k=1..s);