Phoswich detectors for small animal PET have been developed to measure the depth of interaction and to improve resolution performances. The aim of this study was to perform simulations to design the optimum lengths of crystals composing a dual layer phoswich detector. A simulation tool, the Geant4 Application for Tomographic Emission (GATE), was used and validated against a commercially available small animal PET using a dual layer phoswich detector. The newly designed small PET system employed cerium doped lutetium oxyorthosilicate (LSO) --cerium doped lutetium-yttrium aluminum perovskite (LuYAP) phoswich detector modules, which consisted of 8 x 8 arrays of dual layer crystals, with a 2 x 2 mm sensitive area, coupled to a Hamamatsu R7600-00-M64 position sensitive photomultiplier tube (PSPMT). The sensitivities and variations of the radial resolutions were simulated by varying the length of the LSO front layer from 0 to 10 nun, while the total length (LSO+LuYAP) was fixed to 20 nun for 10 and 19 cut diameter ring scanners. The results support the validity of the simulation tool. The radial resolution uniformity was markedly improved using the depth of interaction (DOI) information, and the optimal lengths of the crystal layers for minimizing the variation of the radial resolutions also existed. In the 10 cm ring scanner configuration, the radial resolution was kept below 3.4 mm, over 8 cm field of view (FOV), while the sensitivity was higher than 7.4 % for the LSO 5 mm : LuYAP 15 mm phoswich detector. The resolution variations were minimized to less than 3.6 mm, over 16 cm FOV, using a LSO 6 turn : LuYAP 14 mm phoswich detector for the 19 cm ring scanner. In this study, the optimal length of the dual layer phoswich detector was derived to achieve a high and uniform radial,resolution.