I'm not sure what you mean by controlling the aspect ratio... The controls for "Save As..." are:
Quality: sets how dense the mesh is - "low to high"
"Save normals" check box
"Translate triangles in quadrangles" check box
"Units Digit Multiplier" [1,2,5,10,100,1000]
"Decimal place" [max,2,4,6]
I've posted some obj examples for you to look at.
http://www.pioneercad.com/maxwell/PART/block.zip
block1.obj: mid-quality, quads
block2.obj: mid-quality, triangles
block3.obj: highest-quality, quads
block4.obj: highest-quality, triangles
The exporter recognizes faces based on color. If you make each face of the cube different colors, when its exported you can select each face individually and apply materials at will. I have to admit, I used this when the SW-Maxwell plugin was a little harder to use (I really like it now!). I would assign different color faces to a model and export it as an LWO file. I'd then bring it into Lightwave and apply materials and render Maxwell from Lightwave. I haven't had to do that for awhile now...
The most exciting phrase to hear in science, the one that heralds new discoveries, is not "Eureka!" but "That's funny ..."
Isaac Asimov