/statusbar_selected_item_format=

[Stamimail]

to change it to:

/statusbar_selected_item_format_0= (no object selected)
/statusbar_selected_item_format_1= (1 object selected)
/statusbar_selected_item_format_2= (2 or more object selected)

To determine fixed or "automatic displayed by column", we can use <A>uto flag
<A> </A>

Current:
/statusbar_selected_item_format=1 of $o object(s) selected, size: $s, Date Modified: $m, Path: $p

Will be:
/statusbar_selected_item_format_1=1 of $o object(s) selected, size: $s<A>, Date Modified: $m</A><A>, Path: $p</A>

/statusbar_selected_item_format_0=$o object(s), Total Size: $t

[void]

Added to my TODO list, thanks for the suggestion.