The first digit would have to be 1, the remaining digits, zero.
1023456789
10234567
0123456789
10234567
Since there are only five different digits, a 6-digit number can only be generated if a digit can be repeated. If digits can be repeated, the smallest 6-digit number is 111111.
102
103
If repeated digits are allowed, then the largest 12-digit number is 999,999,999,999 .If repeated digits are not allowed, then the largest 12-digit number is 989,898,989,898 .If the same digit can't be used more than once, then the largest possible number has only10 digits. The number is 9,876,543,210 .
999,999,999,999
You didn't mention whether we're allowed to repeat (re-use) a digit.Here are the smallest 5 primes using only those digits:With repeated digit: 223, 233, 277With no repeated digit: 257, 283
1000000000 or, if negative numbers are permitted, then -9999999999