This study investigated the cage size preferences of juvenile chinchillas using enclosures that allowed free choice between different dimensions. Three comparisons were tested: (1) small floor area (0.15 m²) vs. double floor area (0.30 m²) at constant height; (2) low height (0.4 m) vs. high height (1.0 m) at constant floor area (0.15 m²); and (3) small–low cage (0.15 m² × 0.4 m) vs. large–high cage (0.30 m² × 1.0 m). The juveniles consistently preferred the smaller or lower enclosures across all trials. In the floor area tests, chinchillas spent about 66–75% of their time in the small cage compartments (p < 0.001). In the height comparison, the low cage was preferred by 70% (p < 0.001). When both floor area and height were increased, the small–low cage was preferred by 79% (p < 0.001). Differences in preference were most evident during the daytime period. At night, however, cage utilization was more even. These results indicate that young chinchillas strongly prefer smaller, lower spaces. Based on these preference tests alone, simply providing larger cages (without other modifications) did not result in greater use by juvenile chinchillas. These unexpected results suggest that factors such as safety or familiarity drive juveniles to choose smaller enclosures. Further research is needed to clarify the motivation behind these preferences and to determine how best to incorporate them into improved welfare-oriented housing designs.