When using BitLocker To Go, given that removable drives are inherently offline and may have to resist such attacks, what are the best practices / recommendations for password strength?
In this case, the password takes the role of a symmetric encryption key. So you can use the recommendations for symmetric keys given by various trusted institutions.
For example, if you want to have a password of 128 bits of security, you can generate one of at least 22 alphanumeric characters (a-z, A-Z, 0-9). You can use a password manager for this, to ensure you do not introduce some bias in the generation process. (For the math: 128/log2(26×2+10) = 21.5)