Any boat will hold under anchor if you have the right size anchor.

I'd go with the 12' boat. It's not as big as you may think. It will be fine with one man. Heck, my 16' cat is setup for one man whitewatering for BIG water. It'll hold fine, just get a DB anchor (You'll probably need about a 30# anchor for two). You'll have to experiment with what works. I use about a #40 on my 16' cataraft. I've owned the 9' and have rowed the 12' steelheaders. Am going to build a custom boat on a 12' set of steelheaders soon. So I'd say you'll be fine with a 12' steelheader. A 9' is fine, but only if you never plan to fish with anyone else.
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