How to determine the size of largest site?
How to determine the size of the entire instance?
Courtesy of Todd Klindt here is a PowerShell script to list all databases in your farm in order smallest to largest:
Get-SPDatabase | sort-object disksizerequired -desc | format-table Name, @{Label = "Size in MB"; Expression = {$_.disksizerequired/1024/1024}}
Maybe not exactly what you want, but it might give you inspiration ;-)
Without third-party tools it can be difficult to determine some of the content sizes. You can look at the databases, but that isn't always a clear indication.
I would start by looking at the Quota info available. If it is not configured, or you are not familiar with it, here is something that should help (http://nextconnect.blogspot.com/2009/06/sharepoint-quota-management.html). With quotas turned on you have some reporting at the Site Collection level. It will show how much content is in the site collection and in which content ares (lists, libraries) it is stored.
To get the total picture you would have to review the screen in Central Administration where the quota is set for each site collection. It will show how much is used at the site collection level, and you would have to look at each site collection.