Hi spencerseow
Actually you dont to have to take out the bird to measure the tail lenght.
It will frighten and injure the bird if you are not skillful enough to handle it.

Check with the owner the size of the cage.

For a 9" Shama usually the tailend just manage to touch a bit for a 24" Cage

I got 2 shama one 10" (prawn Tail) and 8" (HardTail) both very active in singing and wagging the tail.

If intested can PM me