diff --git a/tests/phpunit/tests/functions/getDirsize.php b/tests/phpunit/tests/functions/getDirsize.php
new file mode 100644
index 0000000000000..02c19f948978b
--- /dev/null
+++ b/tests/phpunit/tests/functions/getDirsize.php
@@ -0,0 +1,58 @@
+<?php
+
+/**
+ * Tests the get_dirsize() function.
+ *
+ * @group functions
+ *
+ * @covers ::get_dirsize
+ */
+class Tests_Functions_GetDirsize extends WP_UnitTestCase {
+	private $test_dir;
+
+	public function set_up() {
+		parent::set_up();
+		$this->test_dir = get_temp_dir() . 'get_dirsize_test_' . uniqid();
+		if ( ! is_dir( $this->test_dir ) ) {
+			mkdir( $this->test_dir );
+		}
+	}
+
+	public function tear_down() {
+		$this->recursive_rmdir( $this->test_dir );
+		parent::tear_down();
+	}
+
+	private function recursive_rmdir( $dir ) {
+		if ( ! is_dir( $dir ) ) {
+			return;
+		}
+		$files = array_diff( scandir( $dir ), array( '.', '..' ) );
+		foreach ( $files as $file ) {
+			$path = "$dir/$file";
+			is_dir( $path ) ? $this->recursive_rmdir( $path ) : unlink( $path );
+		}
+		rmdir( $dir );
+	}
+
+	/**
+	 * Tests basic directory size calculation.
+	 *
+	 * @ticket 65654
+	 */
+	public function test_get_dirsize_basic() {
+		file_put_contents( $this->test_dir . '/file1.txt', '12345' );
+		file_put_contents( $this->test_dir . '/file2.txt', '1234567890' );
+
+		$this->assertSame( 15, get_dirsize( $this->test_dir ) );
+	}
+
+	/**
+	 * Tests that get_dirsize() returns false for non-existent directories.
+	 *
+	 * @ticket 65654
+	 */
+	public function test_get_dirsize_non_existent() {
+		$this->assertFalse( get_dirsize( $this->test_dir . '/non_existent' ) );
+	}
+}
diff --git a/tests/phpunit/tests/functions/recurseDirsize.php b/tests/phpunit/tests/functions/recurseDirsize.php
new file mode 100644
index 0000000000000..71bfe49a1b452
--- /dev/null
+++ b/tests/phpunit/tests/functions/recurseDirsize.php
@@ -0,0 +1,101 @@
+<?php
+
+/**
+ * Tests the recurse_dirsize() function.
+ *
+ * @group functions
+ *
+ * @covers ::recurse_dirsize
+ */
+class Tests_Functions_RecurseDirsize extends WP_UnitTestCase {
+	private $test_dir;
+
+	public function set_up() {
+		parent::set_up();
+		$this->test_dir = get_temp_dir() . 'recurse_dirsize_test_' . uniqid();
+		if ( ! is_dir( $this->test_dir ) ) {
+			mkdir( $this->test_dir );
+		}
+	}
+
+	public function tear_down() {
+		$this->recursive_rmdir( $this->test_dir );
+		delete_transient( 'dirsize_cache' );
+		parent::tear_down();
+	}
+
+	private function recursive_rmdir( $dir ) {
+		if ( ! is_dir( $dir ) ) {
+			return;
+		}
+		$files = array_diff( scandir( $dir ), array( '.', '..' ) );
+		foreach ( $files as $file ) {
+			$path = "$dir/$file";
+			is_dir( $path ) ? $this->recursive_rmdir( $path ) : unlink( $path );
+		}
+		rmdir( $dir );
+	}
+
+	/**
+	 * Tests basic recursive directory size calculation.
+	 *
+	 * @ticket 65654
+	 */
+	public function test_recurse_dirsize_basic() {
+		mkdir( $this->test_dir . '/sub' );
+		file_put_contents( $this->test_dir . '/file1.txt', '12345' ); // 5 bytes.
+		file_put_contents( $this->test_dir . '/sub/file2.txt', '1234567890' ); // 10 bytes.
+
+		$this->assertSame( 15, recurse_dirsize( $this->test_dir ) );
+	}
+
+	/**
+	 * Tests that recurse_dirsize() respects the exclude parameter.
+	 *
+	 * @ticket 65654
+	 */
+	public function test_recurse_dirsize_exclude() {
+		mkdir( $this->test_dir . '/exclude_me' );
+		file_put_contents( $this->test_dir . '/file1.txt', '12345' ); // 5 bytes.
+		file_put_contents( $this->test_dir . '/exclude_me/file2.txt', '1234567890' ); // 10 bytes.
+
+		$this->assertSame( 5, recurse_dirsize( $this->test_dir, $this->test_dir . '/exclude_me' ) );
+	}
+
+	/**
+	 * Tests the pre_recurse_dirsize filter.
+	 *
+	 * @ticket 65654
+	 */
+	public function test_pre_recurse_dirsize_filter() {
+		add_filter(
+			'pre_recurse_dirsize',
+			function() {
+				return 1234;
+			}
+		);
+		$this->assertSame( 1234, recurse_dirsize( $this->test_dir ) );
+	}
+
+	/**
+	 * Tests that the result is cached in a transient.
+	 *
+	 * @ticket 65654
+	 */
+	public function test_recurse_dirsize_caching() {
+		file_put_contents( $this->test_dir . '/file1.txt', '12345' );
+
+		// First call populates cache.
+		recurse_dirsize( $this->test_dir );
+
+		$cache = get_transient( 'dirsize_cache' );
+		$this->assertIsArray( $cache );
+		$path = untrailingslashit( $this->test_dir );
+		$this->assertArrayHasKey( $path, $cache );
+		$this->assertSame( 5, $cache[ $path ] );
+
+		// Modify file and check that cached value is still returned.
+		file_put_contents( $this->test_dir . '/file1.txt', '1234567890' );
+		$this->assertSame( 5, recurse_dirsize( $this->test_dir ), 'Cached value should be returned.' );
+	}
+}