Yeah, that matte painting is good for getting relative scales, but I know the artist took several liberties with perspective for it - for instance the A-Wings at the back are on a different perspective from the X-Wings to their left, despite being at relative similar depths in the hanger. So, not quite perfect, but close. I'd dig around for reference pics of the studio scale models ILM actually built and filmed for the movies - they were all 1/24 scale I think, so all the New Hope fighter ships were built to the same scale (mostly because they used the same pilot figures).
Also, the life-size Y-wing and X-wing from ANH would be a good place to reference too.
Here's my collection of minifig scale (or close) starfighters.
Hope that helps give an idea of scale.