Sounds like you have a setup very similar to mine. Shoe boxes & sweater boxes. The only thing you didn't mention is what size are the 12 leos now? If they are full grown or near you can move them into the sweater boxes right away. If they are still young I would recommend waiting.
To me the question is not so much the size of the enclosure but communal vs. individual. I keep my hatchlings in individual shoe boxes until they are at least 6 months old (50 grams). This minimizes fighting, biting and STRESS. Leos are NOT social by nature and the only reason to keep them together is for breeding purposes. Just my opinion. If you have the space keep them individually (big box or small does not matter as long as they have smallish hide boxes) until you want to breed them.
Best of Luck!