Everything is changed on the web interface of the printer, this is the same as pretty much any network printer of any brand.
You can change things at the machine on a per scan basis, but changing what the defaults are is done on the web interface (From here on I will refer to it as CWIS).
To make your sizes smaller, you disable the things that make the scan larger. The first thing you would want to do is disable the OCR (Searchable) option, because embedding the fonts increases the size dramatically. And since you don't same to care about print quality and are just wanting good digital look (Small file size is OK on a PC, it is awful on a print)
So take the printers IP address and put it in the address bar of your web browser to load CWIS.
Go to the Scan tab and select the template you want to change on the left (1) then scroll down on the right, increase compression and decrease file size (2) and disable searchable (3). You will also notice many options for encoding, I can't tell you what is best for your usage as I don't know your usage, so you will just need to test it.
Once you figure out what is needed, apply it to the other templates as needed, to make these settings the default for any templates created in the future, just go to CWIS > Properties > Services > Workflow Scanning > Default template and apply these same changes there
Current firmware levels also are able to do some smaller sizes, so update your printer to this version