HFCheung

Photos are excellent but still not perfect.  There are very significant defference in the size between Small Niltava and Large Niltava that you can probably distinguish in a glance, but that may not show well in a photo.  So I guess you need to do more work to judge the size of the bird.  For example, if you can measure the size of the branch that the bird was standing on, or better still, if you have a photo that have a common bird next to it.  Can you do that?

If you prove the size, please submit it to the HKBWS record committee.

HFCheung

If this is only Dark-sided Flycatcher size, so it seems Small Niltava is mmore likely.  I have not realised that female Small and Large Niltava can be so similar before.  I really hope that you can come up with a size similar to a Bulbul.  If size is in doubt, it would be very useful to walk up and measure the size of the branches after the bird has moved.