@Yiwei3 Big bottles, both because of sustainability and to me it gives more of a sense of if not opulence, at least abundance. We use Trader Joe's tea tree oil shampoo/conditioner, it has a nice tingle effect, and is inexpensive, nice packaging. For shower gel we usually use The Body Shop, which we get at Marshall's at a significant discount. We use either Trade Joe's bar soap, also fairly inexpensive or whatever other soaps we find at department stores that are a good value.
Another reason to use larger bottles beyond the environment is that for guests who stay more than 2/3 days you would need to resupply and for people traveling on airplanes they are very, very unlikely to take a huge jug of liquid w/them so less risk of your product disappearing.
ETA we top off the bottles when they get about 2/3 to 1/2 full, so guests are always getting something that is almost full, which is good for presentation. If you use large bottles though you do have to check on them, wipe them down so they stay clean.