I used to change my label size when i went to get it from the eBay get label page and i specified the size as 6x4 so that when i came to print it i could position it on the unused label side. I took it into a graphics programme to do this (i have a paid one but there are free ones like canva and such like) It took me quite a while to figure how my printer printed things, like if i have a free label space on the top right but when i printed it, it came out bottom left. So try a test first on blank paper to see how your printer works and mark the positions.
I will have a look at where i found the setting for it but i think it was easy to find.