It all depends on how much help you think you'll need. Get a bigger band for more assistance. The video shows a medium Iron Woody band. That would give you 50-120 lb of assist (depending on how much it's stretched out) and it's wide enough that it won't roll up or pop off when you're using it. If you're strong enough to only need a miniband (which is only 1/2" wide), then it'd probably be easier to just do negative chin-ups because the band tends to roll off your leg or foot.
Or you could buy one of the packages that have several bands of various sizes. Then you could use a smaller band as you get stronger. They're pretty cool for lots of other exercises too. Dave Schmitz has a lot of information on how to use them up on his site.
This is the light band (which is purple) that I have. It's a similar size to the Iron Woody medium.
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The two companies label them differently and so it's a bit confusing. Iron Woody's calls them (in increasing order of thickness):
mini
super mini
small
medium
large
The JumpStretch bands are very close to the same sizes, just called:
mini
monster mini
average
light
strong