I'd say they're big enough to enjoy an 18 or even a 20. Neither of those, as a jib-and-main boat, is over-the-top as a first boat
A Hobie 20 would be way too much boat for a first cat. It can be a handful for even experienced sailors. The Hobie 18 might be ok as long as he uses reasonable discretion. He also didn't say whether he has any prior sailing experience.
If he's looking for a very basic starter cat to go cruise around on with one or two people, he should consider the Hobie Wave or possibly the Getaway if there could be more than 2 adults. Otherwise a Hobie 16 is a classic starter cat for two adults about his size. A Hobie 18 only if he's up for more complexity and slightly more performance.
sm
