If you start up the tvguide from xbmp, and you do see the channels listed but you don't see any show information, then it's most likely because you never set the date on your xbox, or you unplugged it after setting the date so it got reset.
Look at the date in the upper left corner, and compare it to the dates of the listing you downloaded. If they don't match up, you won't see any shows because you're on the wrong day. You can set your date/time from the MS Dash.
As far as the maximum size of the listing file, you definitely want it below 4 meg I think, below 3 meg is pretty safe.
Currently it's being developed to allow the listing file to be arbitrarily large, but this will come in a later release.