Ok, when I look at zones in Firefox 15.0.1 on Mac OSX 10.6.8, they seem to be consistently way too huge on the map. You have to zoom right out to see them.
For example, when I look at this zone:
http://www.openbmap.org//with_osm6.php5?mcc=234&mnc=30&lac=1082&cellid=723&zoom=17
The zone covers virtually all of England, and stretches right out to Norway (screenshot attached). All the nearby cells from the same network seemed to have the same zone shape and size. Odd. I suspect this is the network extent (with an erroneous GPS point in Norway?!)
When I view this zone:
http://www.openbmap.org//with_osm6.php5?mcc=234&mnc=33&lac=3310&cellid=62596&zoom=13
The zone is displayed as rather large compared to Manchester. This seems unlikely.
Another similar example.
http://www.openbmap.org//with_osm6.php5?mcc=234&mnc=33&lac=1218&cellid=49141&zoom=16
however. Some zones seem feasible sizes, eg:
http://www.openbmap.org//with_osm6.php5?mcc=234&mnc=33&lac=1489&cellid=7117&zoom=12
However, it is possible that this is shown much larger than it should be also.
Screenshot of zone presented for http://www.openbmap.org//with_osm6.php5?mcc=234&mnc=30&lac=1082&cellid=723&zoom=17