Josh Smith is 6-8 to 6-9 and weighs 225 to 230lbs.
He is an undersized power forward (undersized quick four).
Shawn Kemp was 6'10" and 230. He was pretty good in Seattle. The league is smaller then it was in the 90s and early 2000s. Smith can play the 4 based on his size. His shot selection and offensive skill set are what make people feel he is "positionless" at neither being a 3 or a 4.
Playing with Rondo and passers like KG and Pierce can improve that efficiency.
Josh Smith has a position. He is combo forward who is best off as an undersized quick four but can also play some small forward as a power three. A PF/SF. Shawn Kemp was a different type of weapon; an undersized finesse based center / power based power forward. A C/PF. Kemp could play either position dependent on what style of play his team wanted to utilize (speed or power).
Shawn Kemp is more comparable (in terms of position) to an Amare Stoudemire (or Hakeem Olajuwon) type of player whilst Josh Smith is more comparable to an Andrei Kirilenko (or Shawn Marion comparison above) type of player.
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Shawn Kemp was also 240-245lbs during his best years in Seattle. The 230lbs was from early in his career when he first came into the league.
Josh Smith also came in at a lower weight (220lbs) and then beefed up. He was up to around 250lbs two seasons ago and was at 240lbs prior to that ... but then dropped the weight because it wasn't good weight. He (correctly) felt it was holding his game back. He got back down to 225lbs last season and had the best year of his career (with significantly room for more improvement). Allows him more positional versatility and better enables him to out-quick his opponent at PF.
Besides a strength advantage, Kemp was also taller and significantly longer. Josh Smith's reach is about average for a PF whilst Kemp's was more in line with a center. Just a bigger body than Smith has.