For a small bathroom area, the smaller one will do fine.
You may have problems though, due to the excessive amount of layers of shingles on the roof currently.
Follow the instructions on the kit and you will also need to remove and replace the shingles in the surrounding area where the tubular skylight is to be installed.
They work better than I could have ever expected and are very easy to scribe through a drywall ceiling with a keyhole saw and follow the template and just trace out that diameter of a hole.
Sun Tunnel was the brand that I used, but I think that now, Velux bought them out and you can find them on their website, plus I have seen them at the big box stores.
The top portion and the left and right portions of the flashing will require the adjoining shingles to be installed on top of that sheet metAL, but the bottom portion, meaning the section closest to the eave edge, will need to go on top of the existing shingles.
Stripping in the left, right and top portions with some Ice and Water Shield might be a good idea for a DIY project too, just to minimize the potential for leakage.