ClipSize

From DataMystic:

ClipSize is a simple program that shows you how much memory the clipboard is using. This can be very useful when you are using a computer with limited memory or with memory-intensive applications such as image editing or desktop publishing. Most MicroSoft applications give you the option of clearing large amounts of data from the clipboard when you exit them - most other programs do not. ClipSize fills this niche by displaying the current size of the clipboard, and allowing you to clear or view it.