score:6
The official criteria for chargeable luggage carried in the passenger compartment is linear dimension (length + width + height) greater than 140 cm.
From the HK Transport Department's Hong Kong Taxi Service Guidelines (mirror 1, mirror 2), page 10:
Baggage charge may be levied on every piece of baggage that is carried inside the luggage compartment, and every piece of baggage with total dimensions ( length + width + height ) exceeding 140 cm that is carried inside the passenger compartment. The dimensions of baggage should be calculated based on the measurements at the time when they are put into the taxi. Baby carriages are also subject to additional charges for baggage according to the above guidelines.
The luggage must also be able to be safely secured inside the passenger compartment and not damage the vehicle.
Of course, taxi drivers aren't going to routinely use a tape measure on all bags, so I think jpatokal's "looks like a carry-on" is effectively what happens unless you dispute it.
Upvote:5
At the end of the day it's at the taxi driver's discretion (edit: oops, apparently there's an official limit, see the other answer) but a reasonable rule of thumb is does it fit in an airplane's overhead compartment? If yes, it's probably OK; if not, it almost certainly has to go in the back.
Another option is to use Uber, which is quite popular in Hong Kong and is not only (usually) cheaper than taxis, but does not charge for extra baggage.