diff --git a/README.md b/README.md
index 8155285..700d34e 100644
--- a/README.md
+++ b/README.md
@@ -66,6 +66,9 @@ cmake --preset atmega328p-generated
 cmake --build --preset atmega328p-generated
 ```
 
+The firmware is **8380 B** of flash, byte-identical between the generated and
+reflect modes, and `ctest` holds it to that number.
+
 ## Atmel Studio
 
 `master` carries a Studio solution, so this branch does too: `ide/fantemp.atsln`
diff --git a/test/CMakeLists.txt b/test/CMakeLists.txt
index e48917b..030d360 100644
--- a/test/CMakeLists.txt
+++ b/test/CMakeLists.txt
@@ -1,5 +1,9 @@
 libavr_format_test()
 
+# The image must stay below the boot section at 0x7e00, and the README states
+# what it measures; a library advance that moves it says nothing on its own.
+libavr_size_claim_test(fantemp 8380)
+
 # The battery is a compile: a static_assert that fails is the failure. It is a
 # target rather than only a ctest so a plain build catches a regression too.
 add_library(consteval_tests OBJECT consteval.cpp)