Change Values of DropDown List for ArchiveQuota in EAC

You can try with powershell command if you are doing for all users:

Get-Mailbox -ResultSize Unlimited | Where {$_.ArchiveDatabase -like "Archive Database 1"} | Set-Mailbox -ArchiveQuota 2GB -ArchiveWarningQuota 1.5GB

@Saif Shaikh

Thats not the answer to the question I asked :)
I asked how to change the list, I know that I can change the values for all users by that command.

BR
Bastian

As far as I know the drop down list are pre-populated ones in ECP, and that the reason you cannot set your own and hence power-shell command will be useful in changing the quota as per request.

@Saif Shaikh
The strange thing is, we do have several values in the drop down list, depending which database the archiv is located on.

No you can't change the values from the list box, but you should be able to manually type a value other than what is shown. So if you wanted 10 GB and its not in the list, you can just type 10 even though its not listed. Now that is not the case of O365, but if your exchange is on-prem you can manually type a value, not modify values in list.