The last time I checked, the only things still canon at this current time are the moves, clone wars animated series and the first novel released after the canon cut, with plans for other material to be reworked in at a later date.
As for the star destroyer not being an Imperial class, it's hard to tell because the one at the start of Ep IV and ones later in the trilogy share very similar profiles. Even so, the Tantive IV comes in at around 150m. so for that ship to fit in the hanger, that bay would have to be around 175 m to comfortably fit the Tantive IV the way it did in the movie. Now looking at models of the Imperial II class, which fits both the outward and underside profiles of the EP IV Star destroyer, we see that the Star destroyer around 6 hanger bays long, putting the length at 1050m. Now because we are guessing some measurements based on screen appearances. this number could go up to 1200m if the hanger is 200m long, or it could drop to 960m if the hanger is 160m long I personally feel 175 is a nice number for the hanger as it falls into the middle ground of both possibilities.