Hi,
The product quantity must be an integer (a whole number) and it does not support decimal numbers.
So if you want to use such kind of special quantity ; you have to use a fixed unit (like "cm"). And then "5.6m" will become an integer : 560cm
Afterwards, it is possible in HikaShop to have special quantity selectors (we propose several by default).
You can see to override one in order to have you special selector which will convert the user selection to that "integer quantity".
It requires some development skills ; but it is something possible with Hikashop.
Regards,
Jerome - Obsidev.com
HikaMarket & HikaSerial developer / HikaShop core dev team.
Also helping the HikaShop support team when having some time or couldn't sleep.
By the way, do not send me private message, use the "contact us" form instead.