I’m installing kubuntu in a new laptop but I don’t see the option to resize the preinstalled windows' partition
The option is there but might be a little hidden:
If you you are given the option “Install alongside existing OS” (which you might not depending on your existing disk layout) then select the drive you want to install to. Then you can drag the divider on the “After” disk usage indicator to the size you want.
If you don’t have that option then select the “Manual partition” option and click on the partition you want to resize, enter the new size and click “Ok”. DO NOT check the format option or it will erase the data on that partition.
Note that resizing partitions can be considered a dangerous activity and you should always backup any important data before hand.