This question seems to be coming up quite a bit in several of the topics, so I thought it might be useful to give it its own thread.
Is your converter box giving you a small picture inside a big black frame? There is nothing wrong with your box. Here's an English explanation of what's happening, and why you have to use the "Zoom" button on your box's remote to fix it.
Digital TV stations are either 4x3 (standard screen) or 16x9 (widescreen). Only a very VERY few of them actually switch back and forth.
Obviously you've seen, on the analog stations, what happens when a 4x3 station shows a 16x9 source. Letterbox, right?
Well, the stations that are being transmitted with a 16x9 picture do the same thing with 4x3 material, only now the black bars are on the sides instead of top and bottom. We call that "pillarboxed."
OK ... now let's say your station is doing this. The converter box has no way of knowing this is happening. As far as it's concerned, the black bars on the sides are part of the widescreen picture. So what does it do to a widescreen picture? It letterboxes it!
Thus you have bars on the sides, which were put there by the station, and bars top and bottom, which were put there by the converter box. Voila - a fully framed picture, a/k/a postage stamp.
The solution is to tell the box that it's OK to chop off the sides, which you do by pushing the "zoom" button.
None of the actual image is getting cut off if all is functioning as it should. The "zoom" is preset to take that 4x3 postage-stamped picture and perfectly match it to the size of your 4x3 screen, cutting off only the black bars on the sides.
Now, all CRT TVs have a bit of overscan, meaning that about 5% of the picture is intended to be off the sides of the screen. That's part of the NTSC standard, and it's been happening ever since you got the TV. So if you don't see the very edges of the picture that you were seeing when it was postage-stamped, that's why. You won't miss anything of any importance. TV producers all know about overscan.
Now, there is a slight possibility that you have some dang fool local station with a CEO who hates black bars in all forms and has demanded that the engineering department reduce them as much as possible and he doesn't care how they do it. Such stations usually end up broadcasting their 4x3 programming purposely stretched to approx. 14x9. There is also a slight possibility that the station has inadvertently (caution - brief tech speak ahead) set their upscaler's digital NTSC input to square pixels instead of NTSC standard, which will produce a 3x2 picture (also stretched just a bit). In either of these cases, hitting your zoom button WILL shave off a bit on the side edges. This is not the box's fault, though. It's your nincompoop local station, and at this point, there's absolutely no excuse for it. Start making some irate phone calls.