I think its just badly written in the docs.
From dhcp6c.conf(5) — wide-dhcpv6-client — Debian testing — Debian Manpages
sla-id ID ;
This statement specifies the identifier value of the site-level aggregator (SLA) on the interface. ID must be a decimal integer which fits in the length of SLA IDs (see below). For example, if ID is 1 and the client is delegated an IPv6 prefix 2001:db8:ffff::/48, dhcp6c will combine the two values into a single IPv6 prefix, 2001:db8:ffff:1::/64, and will configure the prefix on the specified interface.
sla-len length ;
This statement specifies the length of the SLA ID in bits. length must be a decimal number between 0 and 128. If the length is not specified by this statement, the default value 16 will be used.
So I think the proper format should be:
<format>u32:0-2**128-1</format>
I have no idea on how to define 2^128-1 as a number in the format definition but since u32 is used Im guessing the largest allowable number would then be 2^32-1 which is 4294967295.