You can use the "Select All" entry in File List toolbar, under Main tab ("Edit" menu in the Classic interface) to select all the files listed in the main PowerArchiver window. "Deselect All" deselects all the files and "Invert Selection" deselects the currently selected files and selects the currently unselected files.
You can also use "Select Files" option in File List toolbar, under Main tab ("Edit" menu in the Classic interface). It allows you to select files in the currently opened archive based on several different criteria:
Name
Date
Size
All options are used when searching, so if you enter both a date and size criteria, PowerArchiver will search based on both.