Final answer:
To determine the lengths of DEF in congruent triangles ABC and DEF, each side length of DEF corresponds to and equals the length of its respective side in ABC.
Step-by-step explanation:
If triangles ABC and DEF are congruent, then every side and angle of triangle DEF correspond to the equivalent side and angle of triangle ABC, maintaining the same measurements. To determine the lengths of DEF, one must identify the corresponding sides in triangle ABC and use their known lengths. For example, if side AB in triangle ABC is known to be a certain length, then side DE in triangle DEF, which corresponds to AB, would have that same length. This is due to the fundamental properties of congruent triangles where corresponding parts are equal.