When I added HD digital cable service to my package, the CableCo gave me a Scientific Atlanta Explorer 8300HD box. One of the rather obscure features on that was a zoom function of its own.
My TV had a zoom feature (much like what you've tried in SlingPlayer), and so did the cable box (but I didn't know about that one).
Playing around with the cable box zoom fixed the cropping issue on my TV. There were two buttons on the huge remote, and some stuff to fiddle with in the cable box setup screens. Took about five minutes of play, but I got it sorted out. No more ugly cropping.
This suggestion might not do it for you, but it's worth a look. Check your video source, see if you find an ornery zoom feature.
Good luck !
I had this problem too.
I checked here on the Slingbox forum and found this post/thread.
I checked my cable box settings, mucked around with a few settings, but to no avail.
Finally, I looked at the the Slingbox website, while watching my Slingbox.
I found a "Zoom" button in the bottom right. I clicked on that twice, so that a "D" is displayed, and I got my full screen view!
There are actually several option on this Zoom button. Play with it until you get your desired view.