Tabletop greenhouse

I'd like to build a simple tabletop greenhouse / terrarium? -- maybe 12 x 24 inches or so with some flourescent lighting.
My question is this --
How do you determine the size of the openings to ventilate it?
Is there a specific formula based on cubic feet, etc?
I would imagine it would depend on plant size/species etc but just a good rule of thumb thing that would be good for everything from seedlings to maturity.
I just want to grow simple plants like Basil, etc.
Also place holes to the top or bottom or both? (bottom inlet -- top outlet?)
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