TL;DR: Over-complicate Exchange database selection during mailbox provisioning. For each mailbox in a collection of Exchanges mailboxes, return PrimarySmtpAddress and the smallest database (factoring in AvailableNewMailboxSpace). As databases are selected, add the mailbox's size to the database size so the next database selected factors in the s…

GetMoveParameters.ps1

function GetMoveParameters {
    [CmdletBinding()]
    param(
        [string[]]$Mailbox,
        [string[]]$Databases
    )

    PROCESS {
        if ($Databases) {
            $dbs = $Databases | Where-Object {$_.Mounted -eq $true}
        } else {
            $dbs = Get-MailboxDatabase -Status | Where-Object {$_.Mounted -eq $true}
        }
        $DbSizes = foreach ($db in $dbs) {
            [PSCustomObject]@{
                Name = $db.Name
                Size = $db.DatabaseSize.ToBytes() - $db.AvailableNewMailboxSpace.ToBytes()
            }
        }

        foreach ($mb in $Mailbox) {
            $MbStats = Get-Mailbox $mb | Get-MailboxStatistics
            $MbSize = $MbStats.TotalItemSize.Value.ToBytes() + $MbStats.TotalDeletedItemSize.Value.ToBytes()
            
            $SmallestDatabase = $DbSizes | Sort-Object Size | Select-Object -First 1

            ($DbSizes | Where-Object {$_.name -eq $SmallestDatabase.name}).Size = $SmallestDatabase.Size + $MbSize

            [PSCustomObject]@{
                PrimarySmtpAddress = Get-Mailbox $mb | Select-Object -ExpandProperty PrimarySmtpAddress
                TargetDatabase = $SmallestDatabase.Name
            }
        }
    }
}