Alum is aluminum sulfate. Aluminum reacts with sulfuric acid to produce alum. While the reaction is faster with smaller pieces of aluminum (more surface area to react act), the yield will not be affected as long as there is sufficient sulfuric acid to complete the reaction (even then, the shape and size of the aluminum would not have an appreciable effect).
If you reallllllly want to get into the most picky detail, the aluminum oxide surface layer may not react with the acid, thus favoring the larger piece, which has less oxide on it. This would be an issue only for the most tedious and ridiculous big-time research. If you are asking this question, then this is certainly not an issue for your synthesis, although raising the issue might
If you reallllllly want to get into the most picky detail, the aluminum oxide surface layer may not react with the acid, thus favoring the larger piece, which has less oxide on it. This would be an issue only for the most tedious and ridiculous big-time research. If you are asking this question, then this is certainly not an issue for your synthesis, although raising the issue might