If you're talking about the RIPE region, then; - Can I request a /48 or larger IPv6 prefix from my sponsor LIR -> Yes. Something bigger than a /48 might be hard, but /48 shouldn't be any problems. - Can such IPv6 prefix be further distributed to customers? -> No. A IPv6 PI assignment can only be used by yourself and your own infrastructure and not assigned to customers[0].

Other RIRs will have the information on their website, or your sponsoring LIR will be able to answer any questions :)

[0] https://www.ripe.net/publications/docs/ripe-738 section 7