The larger is about 11 cm from tip to tip and given the curvature on it (which the photo doesn't show) it is almost certainly a primary (probably P4) — you can see the black on the wings feathers (probably P1/P2) of the bird shown below. For a naturally moulted feather it is in remarkable condition, and the green is wonderfully iridescent.
The smaller feather is about 4 cm from tip to tip so is probably a breast feather. It really is that very lemony yellow. There were a couple of parakeets sitting in the top of our silver birch tree while we were sitting outside eating tea and I actually saw this feather floating gently onto the lawn.
(the relative sizes are approximately correct)
Greenford, 7 July 2013
And yes, they are from one of these beauties ...
Greenford, March 2012