motherboard supports max 256 GB SSD?

Solution
IT depends if the how the bios is setup , it could limit down to 256gb . So it would only use a small proportion if you use a bigger drive .
Hi,

There should be no limitation to how much SSD storage size you want to install in your laptop. :)

Here's the complete technical specification of your laptop and again it doesn't say anything about limitation of storage size you can install.
http://support.hp.com/in-en/product/hp-envy-15-w100-x360-convertible-pc/8499294/document/c04951388/