The length of the registration ID is not constant. According to what I've read, its limit is 4096 bytes. However, in practice, all the registration IDs I've seen were much shorter (around 100-200 bytes).
The characters I've seen used in the registration ID are a
to z
, A
to Z
, 0
to 9
, -
and _
. You can store it in any string/character/varchar column.