Dealing with small images in dataset

I am working on a practice problem called Age-Prediction hosted by Analytics Vidhya.

The size of the images in the dataset is quite varied:

I have trained with sz = 16,32,64,100 with resnet50, resnext101 and inceptionv4 but unable to make any significant improvements on my score. Its a 3-class classification problem and I am stuck at the ~0.7 mark on accuracy.

Anyone has ideas on how to get the fastai framework to work with small images but of substantial proportion in the dataset?

The kaggle seedlings competition has the same type of size issue. I just trained at sz=299 and it worked great for me…