Refer to the NRF 51DK Spec page, It can be support Bluetooth 4.2 spec. In the Bluetooth 4.2 spec, The device can support the maximum PDU size of 255 bytes
Where can I find this parameter in the DTM sample code? such as \examples\dtm\direct_test_mode\pca10028\s130\arm5_no_packs\direct_test_mode_s130_pca10028.uvprojx