First find all the files you want to convert and store their filenames in a file

Iterate through that files and make a jpeg from them

Finally move all jpgs to a separate location if necessary

 

Getting the size of all files and folders inside the current directory can be done by typing:

Get filesize in descending order from all files in a recursive folder structure

Another quite handy tool is ncdu. It provides you the disc usage in a graphical way on the shell. Install it like this:

List folders bigger 1Mb and list them in descending order

List the ten largest folders inside a recursive folder structure