Preventing bad proportions
Aside form using actual measures (which is what, being and engineer, I would do for architectural and mechanical designs), there are a few practices that would help an artist keep proportions:
- Always add a "measure of a man" model. That is a humanoid model of the right size and proportion of the avatar in game to use as reference. It does not need to have any detail – all that matters is that it has the proportions you need. You can move it around to see if it fits doors and if windows are at the right height. In fact, you can have multiple (standing vs sitting, child vs adult, and so on). This also means you will not be exporting the whole file, just export a selection.
- Have multiple models that need to work together in a single file. You are exporting selections anyway, thus, if you need multiple things to work together, then work on them together... this ensure they are consistent. This also means that any models that need to be worked together get assigned to the same artist.
- Decide on width/height ratios, document it. In particular, you can use the golden ratio (which is not uncommon in architecture). Make sure that elements such as doors and windows follow the proportions you decided. See also the standard paper sizes (ISO 216).
- Have a minimum thickness for elements (walls and bevels in particular). In games, this could help with visual artifacts (such as shadows effects). It is also vital if you were doing models for 3D printing.
- For modeling complex things, in particular organic things, I would argue for using photo references (or even tangible references). And no, using references is not "cheating".
Also check is the field of view of the perspective camera, if you are working with a different field of view in the editor than in the game, you could be making the wrong judgment.
Addendum: About the "measure of a man" model, sometimes a simple block will do. For most cases a block for the head, a block for each arm, and a couple blocks for the legs is all you need. However, if you have a rigged low-poly model, why not use that?
Checking for bad proportions
Aside from using actual measures and from having a reference model you can use to compare...
Change to orthographic projection to check. I mean, in your 3D editor. While humans are very good at guessing proportions in perspective, we are also capable of tricking ourselves. Which reminds me, do not forget to inspect the model from different angles and zoom levels. However, I am guessing you already do that.
If all else is failing... I suppose the next best thing is to import often as part of the modeling proccess.
