I confirm the problem in Xubuntu 20.04 focal - xfce4-settings-manager 4.14.2
It is possible to enlarge the window, but not to shrink it.
The solution is to edit the conf file ~/.config/xfce4/xfconf/xfce-perchannel-xml/xfce4-settings-manager.xml, and to reduce the "value" in :
<property name="window-width" type="int" value="1000"/>
I confirm the problem in Xubuntu 20.04 focal - xfce4-settings- manager 4.14.2
It is possible to enlarge the window, but not to shrink it.
The solution is to edit the conf file ~/.config/ xfce4/xfconf/ xfce-perchannel -xml/xfce4- settings- manager. xml, and to reduce the "value" in :
<property name="window-width" type="int" value="1000"/>