MFCC in Speech Recognition and ANN
You might try padding the shorter (and even the longer) speech samples with a wide range of different (but realistic) background noise environments to create a larger training set (all to an equal training input size).  This might allow the DNN to be trained to be more robust at inference in noise.

You might even want to add variations of noise and a range of slight time-pitch variations to all your dataset samples to enlarge your training set.