Find the smallest exchange database (this is where you want to create new mailboxes) Use in various other scripts.

ExchShowSmallestDatabase

# Script to return the smallest database on a specified server
# Written by Ben Lye - 20th August 2008
 
# By default the smallest database is determined by summing the size of all the mailboxes in each database.
# Optionally the -edb flag can be specified to make the script look at the EDB file size instead.
# If no server name is specified on the command line one will be prompted for
 
# Usage:
# Get-SmallestDatabase.ps1 [-Server <Server name>] [-edb]
 
# Get the command line parameters
Param ([string]$server,[switch]$EDB)
 
# Load the Exchange 2007 snap-in if they are not already loaded
Add-PSSnapIn -Name Microsoft.Exchange.Management.PowerShell.Admin -ErrorAction SilentlyContinue
 
# Check that the Exchange 2007 snap-in loaded
$snapin = Get-PSSnapin -Name Microsoft.Exchange.Management.PowerShell.Admin -ErrorAction SilentlyContinue
If (-not $snapin) {
       Write-Host "Error: Exchange 2007 snap-in not found" -ForegroundColor "Red"
       Write-Host
       break
}
 
# Prompt for a server name if one wasn't passed in
if (-not $server) {
       $server = Read-Host "Server name"
}
 
# Find any databases on the specified server
$databases = Get-MailboxDatabase -Server "$server" -ErrorAction SilentlyContinue
 
# Error if there are no databases found
If (-not $databases) {
       Write-Host "Error: No databases found for server $server" -ForegroundColor "Red"
       Write-Host
       $break
}
 
If ($databases) {
       # Prepare some variables for storing the name and size of the smallest database
       $smallestdbsize = $null
       $smallestdb = $null
 
       # Loop through each of the databases
       Foreach ($database in $databases) {
 
              If ($EDB) {
                     # Get the size of the .edb file
                     $dbsize = (get-childitem ("\\" + $database.Server + "\" + $database.EDBFilePath.PathName -replace(":","$")) | select-object name,length).length
              } Else {
                     # Get the database size in bytes by summing the size of all mailboxes in the database
                     $dbsize = (Get-MailboxStatistics -Database $database.Identity  | Measure-Object -Property TotalItemSize -Sum).Sum
              }
 
              # Compare the sizes to find the smallest DB
              if (($dbsize -lt $smallestdbsize) -or ($smallestdbsize -eq $null)) {
                     $smallestdbsize = $dbsize
                     $smallestdb = $database   
              }
       }
 
       # Return the smallest database
       $smallestdb
}