How much water capacity do I actually need?

Reviewers in r/bidets generally report that 150 to 200 ml is enough for a single use, so a 300 to 400 ml reservoir handles most situations without a refill. Larger 2 L+ 'tank' models are really aimed at camping, RV, or in-home use without plumbing rather than packing into a carry-on. For airline travel, smaller is almost always better.