No, 48x48 is not necessary. Statically link to the icon in desktop

configure.ac

@@ -77,8 +77,7 @@ AC_CONFIG_FILES([
     src/pam_panic_pw/man/Makefile
     src/pam_panic_config/Makefile
     src/pam_panic_config/applications/Makefile
-    src/pam_panic_config/icons/48x48/Makefile
-    src/pam_panic_config/icons/scalable/Makefile
+    src/pam_panic_config/icons/Makefile
     test/Makefile
 ])
 

src/pam_panic_config/Makefile.am

@@ -1,2 +1,2 @@
-SUBDIRS = applications icons/48x48 icons/scalable
+SUBDIRS = applications icons
 bin_SCRIPTS = pam_panic_config

src/pam_panic_config/applications/pampanic-config.desktop

@@ -18,7 +18,7 @@ Keywords[es]=PAM;seguridad;contraseña;autenticación;usb;
 Keywords[jp]=PAM;セキュリティ;パスワード;認証;USB;
 Exec=sudo pam_panic_config
 TryExec=pam_panic_config
-Icon=pampanic
+Icon=/usr/share/icons/hicolor/scalable/apps/pampanic.svg
 Terminal=true
 Categories=Settings;Security;ConsoleOnly;
 StartupNotify=false

src/pam_panic_config/icons/48x48/Makefile.am

@@ -1,2 +0,0 @@
-appicondir=$(datadir)/icons/hicolor/48x48/apps
-appicon_DATA=pampanic.png