DXF files do not always contain the "units" - it will just have something marked as "150" for example, withouit telling you if it's mm, inches, or miles. You have to set units in Aspire (if it's anything like Vcarve, which it probably is). Assuming that the DXF file also has only the parts, not the panel outline from which they are being cut, you also need to set the panel stock size in Aspire and import the DXF and position it on the stock. Might be where your problems are. Or maybe not, as your original question left out a lot of info - as requested by others!

I echo the "Why MDF?" qiuestion. My first CNC router was also built from MDF. All the structural integrity of cold-rolled cow dung. It were crap. Daily readjustment of all bearings to cope with frame warp, etc. I actually did quite a bit of useful work on it and it taught me a lot (which is why the Mk2 is welded steel). Still crap, though.