When trimming them down to size, is there anything to bear in mind? Or any easy mistakes to make with it?
Check the back of the device and make sure your cuts are outside the area delimited by RST in the North, C2 in the South, and the two dashed lines on the East and West.
Also, will the Trill Bar automatically renormalize its scale 0-1 to the new length, or will that have to be done 'manually' in the code?
No, the device will not know about that. You have to do this in software. If you are using a recent version of the Trill Library you can use the CentroidDetection class to do it for you (you will have to specify the pads that are currently in use).