at least a ten gallon. A fifteen or twenty would be better. the cories and the tetras would very much appreciate proper sized schools 6-8 tetras and 5-6 cories would be a good number to go with your betta in one of those larger tanks.
The cories and the tetras just need lots of space because, even though they are small, they stress out a lot without a proper sized school, and a proper sized school of each needs more space to swim. Your fish will appreciate even just a ten gallon, and at Walmart i've seen entire kits with a filter and hood for jut 30-40 dollars
Some Petcos are also having their dollar per gallon sale right now. You can get a tank large enough for these fish plus some more for a very reasonable price.
Bekah is very right about the schools and the stocking. The tetra need large schools around betta because they can get nippy in small numbers. The more tetra there are, the lower the chance your betta could become a target for aggression.
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