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?)

thanks.