Hey there, @Rob_Binet!
I’m sorry for the late reply. 
Regarding the physical dimensions that our @Captain_WD has already posted, you can also check this thread from the community where you will find a picture of the WD PiDrive and another mobile 2.5" HDD from a laptop being compared.
As for the power supply, the label is right, you can use 5 VDC: 0.55A.
Hope this helps you even a little bit. Good luck! 
SuperSoph_WD
