For the tube, the longer is better because the end is tapered and narrow so there's less chance of your betta getting sucked up (it's amazing what they can fit in). For the flow, check and see if there is a little tab, dial, or knob that regulates the flow. Otherwise, a baffle can be made or a piece of filter sponge can be placed in the outpour.
If he's determined to swim around it, low is definitely better since it's narrower. I know that when I take the bottom part of my filter tube off to clean it, I can see how wide the tube actually is and how easy it'd be for my fish to get sucked up if it weren't for the taper on the bottom part. Anything 1/2" in diameter or more is fair game. Trust me, I know from experience with a siphon hose.
there is no hole at the bottom tho? it has a flat bottom so water gets drawn in through the holes if that makes sense. so no chance of betta swimming up it :) i just thought if it was higher and out the way, he would have abit more room to swim around the bottom of the tank?
sorted the flow out now, collecting betta in 2 and half hours time!
Actually, he'll probably like swimming around it. My bettas love little "obstacle courses" that the plants and things make and weave in and out all the time. Plus, I think you'll find your guy can fit behind the tube easy so it doesn't really get in the way that much. But you choose either way you're most comfortable with.