Don't worry about it too much.
If you are serious about learning fire poi, I would say go ahead and buy a fire rig. I used lots of "practice poi" and never really got anywhere. And what little I did learn didn't really translate when I finally got my first fire rig. I didn't even learn the most basic of moves until I got my chains, and I haven't turned back since.
Some people like shorter poi, some like longer poi. And everyone holds their poi differently. Some wrap the chain around their hand and hold the chain betweeen their thumb and forefinger, so even though they bought the 26" long poi, it looks like 21 to 23 inch long poi when they are spinning. I don't do that, so 26" poi would be way too long for me and my style.
I'm not trying to confuse you, or make you think there's no way to make a decision. I'm just saying that at some point, you will have to just break down and make a decision on length, and if that decision is wrong, you will figure it out over time as you progress and develop your style. You may decide you like the option of having the chain relatively long for big graceful maneuvers, or you may decide you like the poi short enough to do inside maneuvers (possibly a crisper, quicker style with more wraps).
I would personally not buy poi significantly longer than my arms. So if your arms are less than 22", I would discourage you from anything longer than 22" or 23" to get started with **unless** you are pretty sure you may want to wrap the chain around your hand to hold it, instead of simply holding the finger loops.
It's really too bad there are no definate answers to give newcomers, but it's that same frustration that fuels the art and development of individual styles.
In the words of my mother...
"Do something, even if it's wrong!"
