After looking at the photos of the Doug figure that Chaz has shared (see "Farewell Doug" thread) it looks like these will be oversized for OO, and certainly way out of scale for HO. I placed a OO scale figure from Bachmann Branchline in the cab of my Gordon, and the figure's head can just barely poke out from the bottom of the side window. In Chaz's photo, the Doug figure can be seen through the window from about the chest up.

Now, in the "Railway Personnel" pack (the one which Doug is included) there are three taller figures, one of which is "Doug". I have a theory that they were intentionally made much taller to look more appropriate in the engine's cabs- most notably Gordon and Henry, as a standard OO figure looks too small in the cab.

Hopefully Chaz can confirm this and elaborate more if he'd like.