Can anyone tell me if driveshafts are different for each model or if they vary on years?
For example will Impreza fit Legacy or vice versa?
Are the diameters / splines / lengths listed anywhere?

 

Generally, fronts are all interchangeable
Some legacy rear inner joints are different from imprezas, in that the rear diff is slightly different.
For those legacy's that the rear diff is the same as an impreza, then driveshafts are the same fitment
VC LSD diff shafts are different to plate type LSD shafts.
Diameters do vary on some models.