I noticed while setting up a new Mycelium HD wallet, that I'm able to choose 12, 18, or 24 words as backup.
As far as I know from Electrum, 12 words is 128 bits of entropy. Does this mean that in Mycelium, I'm able to decide if my master private key is 128, 192, or 256 bits?
The master private key is always 256 bits. A 12 word backup has 128 bits of entropy.
https://github.com/bitcoin/bips/blob/master/bip-0039.mediawikiThe strength of ECDSA inversion from pubkey is 2^128. So some people (including me) think 12 words / 128 bit is sufficient, and longer wordlists may otherwise lead to people not doing backups.
The default for mycelium is 12 Words. At the first startup, If you import a wordlist from an external source you can chose 18 or 24 words also, with optional password.