Constant ARENA_PAGES 

pub(crate) const ARENA_PAGES: u32 = _ /* 1_048_576u32 */;

The arena’s size in pages, as scheds/include/lib/arena_map.h declares it: address space only, backed by the pages actually allocated. An arena spans at most 4 GiB, which 2^16 pages keep to with 64 KiB pages on arm64.