Tracking is free on USPS first class packages (it is also required). Through stamps.com and endicia.com it’s $1.93 for package up to 2oz with tracking. Media mail is not permitted for electronics, only for actual media like books, tapes, records, and CDs.
You don’t have much choice on label size. It can only get so small. Why not print your labels on regular paper for now, then cut out the label with scissors and tape it on?
Later you can graduate to letter-size adhesive sheets with two labels per page (so, 8.5x5.5" labels). You print one side, then flip the page around to print the other side.
I sold things on eBay before which gave me some practice. Try sending yourself a package or two maybe as practice and to learn the ropes. I’m sure you can figure out the details.