Treemacs is not allowing the resize the file explorer window in vanilla emacs

I’ve installed treemacs on vanilla emacs 27.1

when I did M-x treemacs it opens file explorer in the sidebar.

I want to resize (i.e., expand and reduce) the length of the file explorer sidebar but there is no option to do so.

os details –

$lsb_release -a
LSB Version:    n/a
Distributor ID: ManjaroLinux
Description:    Manjaro Linux
Release:    20.2
Codename:   Nibia

emacs version details –

$ emacs -version
GNU Emacs 27.1

How can I make sidebar resizing happening?