Is it possible to select random user in yt chat

It isn't possible to select a random user because YouTube doesn't expose the list of users in chat. You could leverage the !winner command to select a random recent chatter, with the caveat it only includes recent chatters.

I’d also like to create a similar command, for example:
!happy — the user has 20% happiness
!size — the user is 150 cm tall

But Nightbot needs to mention the username of the person who triggered the command, and give random values within a defined range.
For example, if I set happiness between 1 and 100, and height between 120 and 200.
How can I do that, please?

Check out the examples at <https://docs.nightbot.tv/variables/eval> for how to build a random number result. You can leverage other variables like <https://docs.nightbot.tv/variables/user> to show the person who used the command.