Can I use this sign to say 'I can't stand him'?
No, this sign is only used for the physical action of standing on your feet. To say you can't tolerate or 'stand' someone, you would use a completely different sign, usually involving the thumb flicking outward from the nose or chin.
How do I sign 'stand up' versus just 'stand'?
The sign is the same for both! To emphasize the action of getting up, you can start with your dominant 'V' handshape slightly above your non-dominant hand and bring it down firmly onto the palm to show the completion of standing up.
Does it matter which hand is the 'floor'?
Yes, you should always use your non-dominant hand as the stationary 'floor' and your dominant hand as the 'legs.' This follows the general ASL rule where the dominant hand performs the primary action while the non-dominant hand acts as the base.