media_gallery images are not opening with proper size
For the images to be of same size on every refreshing of page:-


you need to define the image style as per your requirement by Configuration > Media: Image Styles
Add the new style by giving it a name 
Add the effect you want to give
Click on Add button and add the corresponding properties.
Update the style 
Finally when uploading the images,choose the image style for the image ,this would result in the constant size of the image throughout instead of the variable size of the image.