How to plan the portion size for a vegetarian bourguignonne dish?
For bourguignonne, most of the calories are going to be in the noodles anyway.  The beef amounts for bourguignonne are smallish, and the flavorings (mushroom, possibly carrot, broth, wine) are pretty much non-existent nutritionally.

I would just go by mouth-feel.  Cut the aubergine in amounts that approximate the texture and flavor of the original dish, and don't much care about the differences.