I'm not a cryptologist so I'm starting to regret posting in this thread 
I quoted this bit below because I thought it would answer your question about a larger size seed:
I've highlighted the relevant bit. DeathandTaxes is someone who knows what he's talking about.
I quoted this bit below because I thought it would answer your question about a larger size seed:In either case brute forcing the seed would allow you to gain access to all the private keys, where as brute forcing a single private key only gives you access to that key. It is generally speaking academic if the seed has sufficient entropy because keys with 128 bit key strength are considered beyond brute force anyways.
I've highlighted the relevant bit. DeathandTaxes is someone who knows what he's talking about.