What is the first drive when you boot Linux Mint from USB and what is where your Windows is installed doesn't have to be the same. Your windows is far more likely to have been installed on /dev/sdb (an SDD?) and /dev/sda is indeed completely free.
Check /dev/sdb if it has your Windows partitions you can just proceed with using (some) of the free space on /dev/sda.
If something goes wrong you can just restore /dev/sdb from your backup. If you didn't make a backup before starting to try and install Linux Mint, you correctly estimated the value of its (i.e. Win10) availability at what it is worth.