Your best bet is to use the "Something else" option and manually set the partition(s) to use for LL. If you've just got a standard Mint installation, you probably only have a root and a swap partition. Based on what you stated above regarding Windows partitions and my assumption of just a standard Mint install, you probably have Mint's root as /dev/sda5 and swap as /dev/sda6, both of which are within extended partition /dev/sda4. Using that as an example, all you need to do on the partitioning page is:
* choose /dev/sda5 and click "Change" button
* Size = leave same as it already is
* Use as = Ext4 file system
* Mount Point = "/" for root
* Check box to format the partition
* Near bottom of window, "Device for boot loader installation" = /dev/sda.
No need to do anything with swap partition or extended partition. Installer will see and use swap automatically.
P.s. If you're not sure which partition is currently Mint's root partition, log into Mint, open a terminal and enter this command:
lsblk
Then just look under the "MOUNTPOINT" column to see which partition has "/" next to it. If that shows something different than my example, substitute your correct partition in place of my /dev/sda5 example.
