In computer BIOS settings for Intel desktop boards, Aperture Size is a video configuration option that determines the amount of system memory available for direct access by the graphics device. While Aperture Size options may vary by board model, typical choices for Aperture Size include; 4MB, 8MB, 16MB, 32MB, 128MB, and 256MB.
See "BIOS Beep Codes" in the Quick Reference section of Webopedia.
Also see "The System Boot Process Explained" in Webopedia’s Did You Know…? section.