In my experience, I perceive all aircraft quite close to their expected real size with the Index (not with the G2 - last time tried about 2 months ago). However the A320 is really too large to me.
If you’re running SteamVR, the latest beta now adds 2 new settings*:
This adjusts the FOV effectively making possible to reduce it to what you are actually really seeing through the lenses. When using the eye relief and pushing the lenses farther away, you’re seeing less but the game is still rendering unnecessary pixels. The new FOV adjustment is making sure the game only renders the pixels within the FOV you’ve set.
NB: I would have preferred the option to reducing the mask size instead, in order to effectively render less pixels and gain fps. The FOV is instead still making the game rendering as much pixels, but in a narrower region. There is a plus to this: you get higher resolution on a smaller region therefore more legibility (super sampling).
This adjusts the world scale. This is pretty much what this topic is about and it is quite funny flying in a tiny cockpit when set at 50%.
However this is a whole-scale adjustment: everything gets tiny or bigger, not just the aircraft. Therefore, this topic is still valid and a solution to cockpit scaling only is needed. Steam did add this slider and I know for a fact it is not something utterly complex to implement either (a matrix mul per eye). The same could be done at the object level for FS2020, in effectively scaling the aircraft 3D model prior rendering. Nothing much complex in itself either.
As for the A320, I find using SteamVR World Scale around 87% is making it life size to me.
*these might be restricted to eh SteamVR Compositor though.