Hello @AlvaroViudez,
For your Q1 and Q2, please check out this post which has the link to a tutorial for transfer learning. The tutorial has a notebook which initiate a pre-trained model for learning and prediction.
For your Q3, the image size has to follow the requirement by the pre-trained network. if the network is designed for 100 x 100 images, then you need to resize your 50 x 50 images to become 100 x 100.
Raymond