Temporarily, a 10-15 gallon tank will be enough (for fish up to 4-5").
For life, you'll need at least a 40x16x16" tank (40 gallon). Adult Reds on average reach about 10-12" in size (although they can get bigger still), so even just a single Red needs a fairly large tank to move and turn around freely.
Oh, and welcome to PFury