diff --git a/src/modules/partition/tests/1a-legacy.conf b/src/modules/partition/tests/1a-legacy.conf
new file mode 100644
index 0000000000000000000000000000000000000000..f3435a262cffbc7ba60b9acebaee48298cd405c9
--- /dev/null
+++ b/src/modules/partition/tests/1a-legacy.conf
@@ -0,0 +1,2 @@
+---
+efiSystemPartitionSize:          100MiB
diff --git a/src/modules/partition/tests/1b-legacy.conf b/src/modules/partition/tests/1b-legacy.conf
new file mode 100644
index 0000000000000000000000000000000000000000..9792e815579e2374a58dac655a19a7610fbc99b3
--- /dev/null
+++ b/src/modules/partition/tests/1b-legacy.conf
@@ -0,0 +1,2 @@
+---
+efiSystemPartitionSize:          100MB
diff --git a/src/modules/partition/tests/ConfigTests.cpp b/src/modules/partition/tests/ConfigTests.cpp
index 9ef30d720b38bdabc1ef5f025435f832c4bda891..4353d5f8f93f54728fef94514cf0692b32cb7082 100644
--- a/src/modules/partition/tests/ConfigTests.cpp
+++ b/src/modules/partition/tests/ConfigTests.cpp
@@ -9,12 +9,13 @@
 
 #include "Config.h"
 
+#include "core/PartUtils.h"
+
 #include "GlobalStorage.h"
 #include "JobQueue.h"
 #include "utils/Logger.h"
 #include "utils/System.h"
-
-#include "core/PartUtils.h"
+#include "utils/Yaml.h"
 
 #include <QObject>
 #include <QtTest/QtTest>
@@ -31,6 +32,7 @@ public:
 private Q_SLOTS:
     void initTestCase();
     void testEmptyConfig();
+    void testLegacySize();
 };
 
 ConfigTests::ConfigTests() = default;
@@ -38,7 +40,7 @@ ConfigTests::ConfigTests() = default;
 void
 ConfigTests::initTestCase()
 {
-    Logger::setupLogLevel( Logger::LOGDEBUG );
+    Logger::setupLogLevel( Logger::LOGVERBOSE );
 
     // Ensure we have a system object, expect it to be a "bogus" one
     Calamares::System* system = Calamares::System::instance();
@@ -60,8 +62,6 @@ ConfigTests::initTestCase()
 void
 ConfigTests::testEmptyConfig()
 {
-    Logger::setupLogLevel( Logger::LOGVERBOSE );
-
     Config c( nullptr );
     c.setConfigurationMap( {} );
 
@@ -79,6 +79,43 @@ ConfigTests::testEmptyConfig()
     QCOMPARE( gs->value( "efiSystemPartition" ).toString(), "/boot/efi" );  // Default
 }
 
+void
+ConfigTests::testLegacySize()
+{
+    Config c( nullptr );
+
+    const auto* gs = Calamares::JobQueue::instanceGlobalStorage();
+    QVERIFY( gs );
+
+
+    // Config with just one legacy key
+    {
+        const auto file = QStringLiteral( BUILD_AS_TEST "/1a-legacy.conf" );
+        bool ok = false;
+        c.setConfigurationMap( Calamares::YAML::load( file, &ok ) );
+
+        cDebug() << "Tried to load" << file << "success?" << ok;
+
+        QVERIFY( ok );
+
+        QVERIFY( gs->value( PartUtils::efiFilesystemRecommendedSizeGSKey() ).isValid() );  // Something was filled in
+        QCOMPARE( PartUtils::efiFilesystemRecommendedSize(), 100_MiB );  // From config
+        QCOMPARE( PartUtils::efiFilesystemMinimumSize(), 100_MiB );  // Taken from config
+    }
+
+    // Different legacy key value
+    {
+        bool ok = false;
+        c.setConfigurationMap( Calamares::YAML::load( QStringLiteral( BUILD_AS_TEST "/1b-legacy.conf" ), &ok ) );
+
+        QVERIFY( ok );
+
+        QCOMPARE( PartUtils::efiFilesystemRecommendedSize(), 100000000 );  // From config, MB
+        QCOMPARE( PartUtils::efiFilesystemMinimumSize(), 100000000 );  // Taken from config
+    }
+}
+
+
 QTEST_GUILESS_MAIN( ConfigTests )
 
 #include "utils/moc-warnings.h"