> couldn't find any way to increase the delay to recognize a double-click in this OS, so my question is, there is a way to change the behavior of Horizon Client to open the remote app with a single click ?
No, it's not supported.
> Another question is, there is any way to increase the menu bar size? (without changing the OS resolution).
Which menubar? ThinClient's menu bar or remote app's menu bar?
You may increase the scale of Linux OS.