No.
In my opinion, knowing the length of the key is not very important. Even if we knew the length, we still might have to go an extra mile and try every possible letter combination which can be tedious since it is a very large number of possibilities. Assume the key length is 20 letters, then there are 26^20 possible keys.
Increasing the size of the key can make the Vigenere cipher hard to crack and thus YES; the larger the key, the better the encrypted message.
