[V4L] window sizes using a mmap interface

I'm playing with v4l using a hauppauge card (WinTV).

I'm bring up a TV display using the mmap interface. How do I know that
the tv window size that I requested is wrong? --By the tv window size,
I mean the size of the image written by video4linux to the mmaped buffer
when the:

ioctl(vf, VIDIOCMCAPTURE, &video_mmap); call is made.

Although the image is written, ioctl returns a -1 error, and the "errno"
value reports an error of (int) 16.

Jamie