How can I determine the space used by contacts on my Android device? How can I see how much space my contacts data is using on Android, including pictures? I have quite a lot contacts (~2500).

I have four android devices (2.1, 2.3, and 4.0.4). Only one of those is rooted.
Go to the Settings app (not the settings in the Contacts app). Go to the list of apps (Apps or Applications manager) and change from Downloaded to All. Find an app in the list called Contacts storage and click on it. There you can see how much storage it's using. Since this app only stores your contacts, all that storage is being used for your contacts. (Actually, it takes up some space even when you have no contacts, but only about 4kB.)