Weight file use for different size image
If your model is fully CNN, there is absolutely no need to have different models. 
A CNN model can take images of any size. Just make sure the input_shape=(None,None,channels)
You will need separate numpy arrays though, one for the big images, another for the small images, and you will have to call a different fit method for each.
(But probably you will be working with a generator anyway)
I want to train CNN where image-dimension is 128*512, then I want to use this weight file to train other data which has 128*1024 dimension. That means I want to use pre-trained weight file during the training time of different data(128*1024).
Is it possible or How can I do it?
I want to do this because I have only 300 images which have 128*1024 dimension, while I have 5000 images which have 128*512 dimension and both datasets are different.
Thank you