I have never had a problem with socks or slippers as gifts and my thoughts on this are easy:
For a size 6 - I knit the foot so it measures 6" from the heel to just before the toe decreasing. Then do the decreases, which are about 2" worth of knitting and you have a socks or slipper that fits perfectly.
Whatever size foot you are knitting for do that many inches from the heel to just before the toe decreasing. This has worked for me from a size 5 to a size 10, men or women.