[Mp4-tech] How do we select bliock size in h.264 during Motion Estimation ?

[Anand Paul]

hi all,
              i am starter for h.264, as we all know we would choose
the block size (i.e 16X16, 16X8,8X16,8X8,8x4,4X8 and 4X4 ) depending
up on the redundancy or complexity of the image. Say for instance
image area with less redundancy may prefer in for 4x4 whereas image
area with more redundancy may prefer 16x16 for efficitent coding.
My question is how do we choose this block size ?, in realtime as we
don't know what would be the next image(frame) look like., in this
case how do we choose 16x16 block or 4x4 for motion estimation.
expecting ur comments 
Anand P.