Solved by JuanFMontesinos in post #15 
              
              
                Soo the point is that original vgg was using that size. Network is “used to see” objects whose sizes are contained in a 112x112 image. 
There is something called receptive field (rather than boring you with a shitty explanation I will link to a blog https://towardsdatascience.com/understand-local-re…