10 gallon minimum, 20 gallon ideal You should be able to pick up something in that size pretty cheaply second hand, if you don't mind it being a tad on the tatty side
It'll need a heater and filter, which will need to have some mature media from the main tank put in it so the tank is cycled. You don't need air bubbles, but an air powered box filter would be a really good choice, if you already have an air pump you can use, as it won't suck up any fry.
A sprinkle of sand on the bottom, but not too much as you'll be having to do a lot of water changes. Bare bottom would probably be best, but some fish don't like it.
Some plants for hiding places would be good; live ones if you can
You could always use a large plastic storage box, if it's easier and cheaper for you (I know money's tight for nearly everyone right now!), the fry won't mind.
I kinda gave up on live plants. it seems that no matter how many I get or how often I buy them, the platys and swords devour them before they have a chance to grow...