I have a callback on the "handoff" signal of fakesink, in a pipeline (see below) that decodes video. 
The problem is that GST_BUFFER_SIZE on the recieved buffer varies instead of having the same constant size as expected by width*height*bpp. The variation is: width*height*bpp + a few dozen bytes, with the "extra" number varying from 0 to a few dozen.

This is a version of gstreamer built using OABuild2 (the build was given to me by the maintainer of http://people.collabora.co.uk/~oleavr/OABuild/)

Using the following pipeline:

udpsrc port=%d caps=\"application/x-rtp, media=(string)video, clock-rate=(int)90000, encoding-name=(string)H263\" ! rtph263depay ! ffdec_h263 ! ffmpegcolorspace ! video/x-raw-rgb,depth=(int)24 !  fakesink name=sink