How to set paper size? Why printing extra blank labels?

You need to set the right paper size to print.

Normally, shipping labels size is 100x150mm/ 4x6 inch.

For labels of other, set the actual width and length, before printing, set the printing scale as "Fit to paper", then print.

Also, you may try calibrate the paper by pressing the FEED button until it beeps once (approximately 3 seconds). Calibrate each time change the paper.