diff --git a/tests/Modifiers_-_--size_from_file_size.sh b/tests/Modifiers_-_--size_from_file_size.sh
new file mode 100644
index 0000000..ecfa36b
--- /dev/null
+++ b/tests/Modifiers_-_--size_from_file_size.sh
@@ -0,0 +1,49 @@
+#!/bin/sh
+#
+# Similar to the "--size" check but, instead, using another file's size with
+# "--size @".
+
+# Dummy assignments for "shellcheck".
+testSubject="${testSubject:-false}"; workFile1="${workFile1:-.tmp1}"; workFile2="${workFile2:-.tmp2}"
+
+# Process 100 bytes at 100 bytes per second, interval 0.1 seconds, with the
+# size set from a file of $1 bytes and expecting a percentage of $2 at the
+# end.
+sizeCheck () {
+	# Make a file of size $1 bytes, for reference.
+	dd if=/dev/zero bs="$1" count=1 2>/dev/null >"${workFile2}"
+
+	# Run 100 bytes through the program, using the reference file's size
+	# as the percentage reference.
+	dd if=/dev/zero bs=100 count=1 2>/dev/null \
+	| "${testSubject}" -s "@${workFile2}" -n -i 0.1 -L 100 >/dev/null 2>"${workFile1}"
+
+	lineCount=$(wc -l < "${workFile1}" | tr -dc '0-9')
+	finalLine=$(sed -n '$p' < "${workFile1}")
+
+	# The number of output lines should be >8 and <13, and the final
+	# percentage should be $2.
+	#
+	if ! test "${lineCount}" -gt 8; then
+		echo "(reference size=$1) fewer than 9 output lines (${lineCount})"
+		exit 1
+	fi
+	if ! test "${lineCount}" -lt 13; then
+		echo "(reference size=$1) more than 12 output lines (${lineCount})"
+		exit 1
+	fi
+	if ! test "${finalLine}" = "$2"; then
+		echo "(reference size=$1): final percentage was not $2 (${finalLine})"
+		ls -l "${workFile2}"
+		cat "${workFile1}"
+		exit 1
+	fi
+}
+
+sizeCheck 100 100
+sizeCheck 200 50
+sizeCheck 50 200
+
+exit 0
+
+# EOF