Some sizes are not displayed in the widget list
Widget sizes that can be used vary depending on the model of the phone and home app.
(Example. For Xperia XZs and Xperia Home, the size is up to 5 x 5)
If the size you want to use is not displayed in the selection field of the widget, check the available size in the instruction manual of the used phone, please.
Also, try to see if 6x6 can be used on a tablet. please.