I honestly cannot explain why .png exported textures give problems as I never bothered to find out once I realised .bmp and .jpg worked everytime.
If you cannot establish the units used in Milkshape then a few tests can be done to determine what works best, I would assume cm's will work as the app is European after all.
Normally when exporting a model you have a settings drop box where it can be selected, however that does not mean SU will always import it.