5 galllon is fine. Get a filter and a heater.
@black_cat is more helpful than me.
5 gallon tank minimum but personally I prefer 10 gallons. Checklist:
-Tank
-Lid (bettas are jumpers!!)
-heater- get a submersible one, not an undergravel one
-filter- usually the hang on back filters are good and some have an adjustable flow, which is good for bettas
-light- if your tank will be near a lamp or window, then you're good, but if not you'll need a light. Turn it off at night because it's good for them to have a natural day/night cycle.
-substrate- don't get anything painted, like those big glass beads and marbles. Personally I'd go with a natural gravel or sand substrate, or a black one, because I"m a fan of natural tanks but suit yourself.
-water test kit- lets you make sure that your tank is cycling properly
-water conditioner- you need to dechlorinate water before putting it in a tank. I use seachem for my hermit crabs.
-decor
-get at least one place for the fish to hide. Make sure that it has multiple exits and it is large enough that the fish cannot get stuck. Make sure that there are no sharp edges, because bettas have delicate fins.
-plants. I prefer live plants. some of the easiest are anubias and java fern. If you don't want live plants, be sure that any fake ones that you get are silk, not plastic. Plastic can tear fins.
-betta leaf hammock- optional, but bettas love them!! These are fake leaves with suction cups. Stick them near the surface of the water. fish love to rest on them!!
-betta log- optional, but again, bettas love them!! These are logs that float half in, half out of the water. Fish love to swim through them and rest in them.
optional: beneficial bacteria can help you jump start cycling your tank.
Fish- get a fish in a clean cup (try to find the cleanest one, I know that they're usually filthy). if you wiggle your finger, it is good if they follow it and very good if they flare their fins at you. I'd avoid crazy tails like halfmoons and dumbos, because they are prone to fin rot.
LIES
you were misinformed