The best is to try boards if you can before buying.
I managed to try an old longboard, a Large Start and a 139 l Go. I could easily ride the 3. The rest of the family is much lighter and could well balance on the 70 cm longboard too. I also learned that the Start is great for adult total beginers (they need super stability), but are very slow and not nice in low wind ,and too large on high wind. Not a board that I would keep. I also learned that the Go is a nice board, but it has to go also on top of the car when the family is with me. Living in a low wind area, the longboard made sense for us. If you live in a medium wind area, maybe you're better with a shortboard and the Go 151 will be perfect.
For a longboard, the longer the better since it is faster in low wind. Width of around 70 cm is faster if you have previous experience. On my shortlist was the Phantom 320 and the Kona One. The Phantom is better equiped with a mast track, more strap positions. I guess it is faster. Look for the videos on this forum by ZedZed, it will give you a good idea of the board capability. The Kona One is probably more maneuvrable, and also simpler to use. Is is covered by a soft deck. I had a great deal for a slightly used one so I went for it. I just love my Kona, it's perfect for us. Unfortunately, I have never ridden another modern longboard. They are very rarely seen on the water around here.
Again, test whatever roughly suitable board on the water. You will learn from this and will be able to make a perfect choice for you.