What is the maximum prefix size routed in the IPv6 Internet? In v4, most of the operators filter BGP updates for prefixes longer than a /24, to preserve the size of the full view. What about IPv6 ? Is there such "convention" that stands out ?

These days it's usually at /48, and it's very unlikely to ever be longer.

There are still AS' filtering longer than /32 though, but they should all have a default at this point (Verizon were a hold-out, but I believe they've given up).

There are also AS' filtering on allocation lengths, so if you got a /32 they might require you advertise it as such (based on the various RIR's documented allocation policies).