While we have tried to make sure that it can cope with embedded
whitespace and other special characters, it will not work with
multi-line data.
+
+Rather than tracking the performance by run-time as `test_perf` does, you
+may also track output size by using `test_size`. The stdout of the
+function should be a single numeric value, which will be captured and
+shown in the aggregated output. For example:
+
+ test_perf 'time foo' '
+ ./foo >foo.out
+ '
+
+ test_size 'output size'
+ wc -c <foo.out
+ '
+
+might produce output like:
+
+ Test origin HEAD
+ -------------------------------------------------------------
+ 1234.1 time foo 0.37(0.79+0.02) 0.26(0.51+0.02) -29.7%
+ 1234.2 output size 4.3M 3.6M -14.7%
+
+The item being measured (and its units) is up to the test; the context
+and the test title should make it clear to the user whether bigger or
+smaller numbers are better. Unlike test_perf, the test code will only be
+run once, since output sizes tend to be more deterministic than timings.