Interested in knowing where to set max size 100kb for example.
Not size in width or height.
Because that is not the way it works. If you like, you can go into the code and set a jpeg quality level in percentage, that controls the amount of compression in the file; too high compression does throw away details in the image. Pixel size does translate into a rough file size. Also, if you are concern with storage space, then you need to edit the code and not store the original file; it is the original file that eats the storage space. Go visit the link posted above.