diff --git a/board/build.mk b/board/build.mk
index aee724c19044c1..9f2c4250c2c772 100644
--- a/board/build.mk
+++ b/board/build.mk
@@ -1,4 +1,4 @@
-CFLAGS += -I inc -I ../ -nostdlib -fno-builtin -std=gnu11 -O2
+CFLAGS += -I inc -I ../ -nostdlib -fno-builtin -std=gnu11 -Os
 
 CFLAGS += -Tstm32_flash.ld
 
@@ -51,6 +51,8 @@ obj/$(PROJ_NAME).bin: obj/$(STARTUP_FILE).o obj/main.$(PROJ_NAME).o
 	$(CC) -Wl,--section-start,.isr_vector=0x8004000 $(CFLAGS) -o obj/$(PROJ_NAME).elf $^
 	$(OBJCOPY) -v -O binary obj/$(PROJ_NAME).elf obj/code.bin
 	SETLEN=1 ../crypto/sign.py obj/code.bin $@ $(CERT)
+	@BINSIZE=$$(du -b "obj/$(PROJ_NAME).bin" | cut -f 1) ; if [ $$BINSIZE -ge 32768 ]; then echo "ERROR obj/$(PROJ_NAME).bin is too big!"; exit 1; fi;
+
 
 obj/bootstub.$(PROJ_NAME).bin: obj/$(STARTUP_FILE).o obj/bootstub.$(PROJ_NAME).o obj/sha.$(PROJ_NAME).o obj/rsa.$(PROJ_NAME).o
 	$(CC) $(CFLAGS) -o obj/bootstub.$(PROJ_NAME).elf $^