I needed to figure out how much space was required to maintain 7 days of event log entries for the Security event log on my SQL Servers. Basically, this is a simple calculation where you check the size of the current event log, find the earliest event, and calculate how many hours of coverage you have. You then extrapolate that to 7 full days (168 hours). It's a simple script and I'm sure it could be more efficient, but it works.

$computer = "MyComputer"
$cumulativeSize = 0
$numDays = 0
$secSize = 0
$earliestDate = get-date 12/31/2020
$colLogs = get-wmiobject -class "Win32_NTEventlogFile" -namespace "root\CIMV2" -computername $computer
foreach ($objLog in $colLogs) {
if ($objLog.LogFileName -eq "Security")
{
$secSize = $objLog.MaxFileSize / (1024 * 1024)
$colEvents = get-eventlog -log security -computername $computer
$objEvent = $colEvents | select-object -Last 1
#foreach ($objEvent in $colEvents)
#{
#if ($objEvent.TimeWritten -lt $earliestDate)
#{
$earliestDate = $objEvent.TimeWritten
#}
#}
}
else
{
$cumulativeSize += $objLog.MaxFileSize
}
}
$currentDate = get-date
$hourDifference = ($currentDate - (get-date $earliestDate)).TotalHours
$logSize = 168 * $secSize / $hourDifference
if ($logSize -lt $secSize)
{
$logSize = $secSize
}
write-host "Computer: ", $computer
write-host "Old Security Event Log size: ", $secSize
write-host "Earliest Security Event:", $earliestDate
write-host "New Security Event Log size: ", $logSize
write-host "Total size of other Logs: ", ($cumulativeSize / (1024 * 1024))

http://www.sqlservercentral.com/blogs/brian_kelley/2010/11/17/powershell-calculating-seven-day-retention-for-security-event-logs/

Powershell: Calculating Seven Day Retention for Security Event Logs

2010/11/17