It really depends more on how much of your HD is filled, and how often you add new larger files. For example, someone might be using only 40GB right now, but if they add or edit new files of 100MB everyday, then Time Machine will need ~3GB/month, or 36GB/year. Add the 40 from the base, and the total is above 76GB. Now take into account how long drives normally last. Since most externals can be expected to last about 4 years or so (depending on how you run them and the quality), it would be wise to get an external that is at least 188GB (the amount that would be used in storage after 4 years). Seeing as how some space is lost due to the whole 1024 v 1000 conversion, a 250GB would probably be best. Again, this is for a hypothetical person. Find out what your brother's needs are and decided accordingly.