I see from the code that the size is in bytes. Here's a doc patch:
--- /usr/local/lib/perl5/site_perl/5.8.0/Test/File.pm Mon Sep 23
14:02:00 2002
+++ test_file.txt Mon Dec 1 16:49:31 2003
@@ -154,6 +154,8 @@
Ok if the file exists and has SIZE size (exactly), not ok if the
file does not exist or exists with size other than SIZE.
+SIZE is in bytes.
+
=cut
@@ -189,6 +191,7 @@
Ok if the file exists and has size less than or equal to MAX, not ok
if the file does not exist or exists with size greater than MAX.
+MAX is in bytes.
=cut
@@ -224,6 +227,7 @@
Ok if the file exists and has size greater than or equal to MIN, not ok
if the file does not exist or exists with size less than MIN.
+MIN is in bytes.
=cut