Aucky - Have a look at photos of quite a few of the UK Police aircraft, as mentioned by
MG. Many of them have a sliding window (mostly left hand, although it can be fitted either side) which opens, without speed restriction, to provide a clear area of 18" high by 10" fore/aft. (480X320mm)
I don't know the make I'm afraid and, whilst it's not big, it's as large as the window can fit.
Helitech - a couple of years ago:
. . . sorry - tried to embed photo but can't, for some reason - try this link
Photograph of Aircraft G-SUFK
