Depending on what you want to do, you may want to consider a big board. For St Kilda a wide board is probably better, but I'm just putting up the other case for people from other areas who may be reading this and to balance the issue.
Sure, there are horror stories of big boards - but they are ones without decent sails and gear. We've taught dozens of people to windsurf on big boards, with a very high rate still in the sport and loving it. We get EVERYONE sailing at least a bit within 2-20 minutes, from grandmothers to 8 year olds. Some of them have done worlds. Some are now instructors. Some run clubs. Some now sail waves. They all love it.
Big boards plane, and plane well - the original Windsurfer can hit 25 knots when sailed right. They are much, much faster than a Go or any shortboard in light winds, and most of the time the winds are light. An original Windsurfer is often faster than the Olympic RSX or any shortboard. They are tough and go upwind very well.
On the other hand, they are tippier, heavier, and are harder to sail in strong winds. They don't jump, they don't gybe as well at high speeds, they don't give you the same high wind blasting sensation. They don't go as fast or as easily in full-on planing conditions.
When we share weekend regattas with modern Formula/Slalom boards, we get to sail twice as many days, they get to go twice as fast at their peak speeds but spend a fair bit of time waiting for wind. Take your pick about which one suits you.
It gets down to factors like how often you'll go for a sail, whether you can go down when it's windy or have more limited options. Where you sail matters too. Around Sydney, the biggest club and hte biggest commercial operation have both given up on Gos because they won't get upwind very well, and they sail in crowded areas.
So look for a Go if you're aiming to get into high wind blasting and that's realistic for you, but it's not the only option. And like others here have said, avoid a skinny old slalom board and stick with something either long or wide.
About the comment "if their sails look like saggy triangles, their boards are over 3m long, have more than 4 footstraps, and a centreboard, find someone else!"
Why? They may like sailing in all conditions, so they like longboards which go better in light winds. They may like racing, where longboards dominate in many places. They may like going upwind in light/moderate winds, where centreboards rule. They may like "saggy triangles" because they're so easy to rig, so tough, and actually out-perform flat sails a lot of the time.
It's interesting that you're telling them to stay away from someone like Jessica Crisp, Olympic Rep and former World Cup champion!
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