I agree that the quota field should be pre-filled (actually I wonder why it isn't and would call that a regression).

However, I don't agree with introducing a new config option for this - we already have enough config options ;-) and this isn't a case of "one size fits all [domains]". Maybe using the maximum allowed quota as default.would make sense? Typical users don't want less anyway ;-)

Hint: updateMaxquota() updates the description text with the maximum allowed quota, also updating the default there should be easy.

In some cases (eg on VPS or outdated hardware (sad but true)) mail admins don't want grant max quota to all mailboxes, and also want pre-filled some quota at every mailbox creation. In that cases I'm apply this patch manually.

I'm agree - one size to all domains not best solution. Maybe better add this as per-domain option, like a maxquota option? But it's requires more time.

per-mailbox max quota would be a good value to pre-fill the field with imho. if I understand correctly there is already this value available, either as the default per-mailbox setting in $CONF['maxquota'], or set per-domain (probably in the database)