Hi Juanito
Seems to work OK on TC10 x86 using Firefox:
tc@E310:~/x$ gsettings get org.gtk.Settings.FileChooser window-size
(968, 689)
# File->Open , resized file chooser dialog.
tc@E310:~/x$ gsettings get org.gtk.Settings.FileChooser window-size
(968, 496)
tc@E310:~/x$ gsettings set org.gtk.Settings.FileChooser window-size '(800,600)'
tc@E310:~/x$ gsettings get org.gtk.Settings.FileChooser window-size
(800, 600)
# File->Open
tc@E310:~/x$ gsettings get org.gtk.Settings.FileChooser window-size
(800, 600)