While it is hard to regulate, I did place a 50lb block of dry ice into my grow closet, which at the time was 2'x3' with ten plants in there, and for two days it showered my babies with co2, of course at lights out, I placed the dryice in a cooler for safe keeping. I would do it again too.
Of course the problem with dryice is price, if you buy it retail they will try to rip you off to the tune of $75.00 but at wholesale the price is about $15.00 for a 50 lb block. As a general rule, dry ice will sublimate at a rate of five to ten pounds every 24 hours in a typical ice chest.