This is a morning shot of what is next to my bedroom window, where I am looking out from my computer right now. The plant in the center is a Dioon spinulosum cycad. Like all cycads, they are miniature plants that grow very slowly. So this one is fairly large. But it will get bigger in the next 100-150 years, and hopefully I will live to see some of it.
Next to it, at left, is the new sago palm (cycas revoluta) that I rescued. As you can see, I only left three fronds, so it will look a whole better when it starts to grow, which I am hoping will begin later this spring, or at the latest, next year. The leaves of cycads are not as sharp as cactus, of course, but they're sharp and hard, and not good to have next to where people might be jabbed by them. That doesn't keep landscapers from putting them in inappropriate places, next to sidewalks, etc. They just have to be ripped out at some point, which is a shame. Actually that's why this one is here, it outgrew its space, so I really should complain, should I? It will also grow for several hundred years, very, very slowly, and I want to see it do its best.
The cycad at left is a Macrozamia Mooreii. It also grows fairly large, for a cycad, and is spikey, too.
When asking about mature sizes of plants, you have to be realistic about cycads. Yes, they will grow tall in hundreds of years, but that's no reason not be optimistic. Give them some room - and then live for a long time!