diff --git a/doc/configuration.rst b/doc/configuration.rst
index be5f8156..06c8e797 100644
--- a/doc/configuration.rst
+++ b/doc/configuration.rst
@@ -765,6 +765,15 @@ Options
     ``optional`` variants. By default only variants with type ``variant`` or
     ``layered-product`` will get ISOs.
 
+**iso_size** = 4700000000
+    (*int|str*) -- size of ISO image. The value should either be an integer
+    meaning size in bytes, or it can be a string with ``k``, ``M``, ``G``
+    suffix (using multiples of 1024).
+
+**split_iso_reserve** = 10MiB
+    (*int|str*) -- how much free space should be left on each disk. The format
+    is the same as for ``iso_size`` option.
+
 .. note::
 
     Source architecture needs to be listed explicitly.
diff --git a/pungi/phases/createiso.py b/pungi/phases/createiso.py
index 1823f15d..e8c97b6b 100644
--- a/pungi/phases/createiso.py
+++ b/pungi/phases/createiso.py
@@ -32,7 +32,7 @@ from pungi.wrappers.kojiwrapper import KojiWrapper
 from pungi.phases.base import PhaseBase
 from pungi.util import (makedirs, get_volid, get_arch_variant_data, failable,
                         get_file_size, get_mtime)
-from pungi.media_split import MediaSplitter
+from pungi.media_split import MediaSplitter, convert_media_size
 from pungi.compose_metadata.discinfo import read_discinfo, write_discinfo
 
 
@@ -284,19 +284,20 @@ class CreateIsoThread(WorkerThread):
 
 
 def split_iso(compose, arch, variant):
-    # XXX: hardcoded
-    media_size = 4700000000
-    media_reserve = 10 * 1024 * 1024
+    """
+    Split contents of the os/ directory for given tree into chunks fitting on ISO.
 
-    ms = MediaSplitter(str(media_size - media_reserve), compose)
+    All files from the directory are taken except for possible boot.iso image.
+    Files added in extra_files phase are put on all disks.
+    """
+    media_size = compose.conf.get('iso_size', 4700000000)
+    media_reserve = compose.conf.get('split_iso_reserve', 10 * 1024 * 1024)
+
+    ms = MediaSplitter(convert_media_size(media_size) - convert_media_size(media_reserve), compose)
 
     os_tree = compose.paths.compose.os_tree(arch, variant)
     extra_files_dir = compose.paths.work.extra_files_dir(arch, variant)
 
-#    ti_path = os.path.join(os_tree, ".treeinfo")
-#    ti = productmd.treeinfo.TreeInfo()
-#    ti.load(ti_path)
-
     # scan extra files to mark them "sticky" -> they'll be on all media after split
     extra_files = set()
     for root, dirs, files in os.walk(extra_files_dir):
diff --git a/tests/test_createiso_phase.py b/tests/test_createiso_phase.py
index 837d2e54..b5412cc9 100755
--- a/tests/test_createiso_phase.py
+++ b/tests/test_createiso_phase.py
@@ -603,6 +603,34 @@ class SplitIsoTest(helpers.PungiTestCase):
                                             os.path.join(base_path, 'Packages/spacer.rpm')])
         self.assertEqual(data[1]['files'], [os.path.join(base_path, 'Packages/x/pad.rpm')])
 
+    def test_can_customize_reserve(self):
+        compose = helpers.DummyCompose(self.topdir, {'split_iso_reserve': 1024 ** 2})
+        helpers.touch(os.path.join(self.topdir, 'compose/Server/x86_64/os/.treeinfo'),
+                      TREEINFO)
+        helpers.touch(os.path.join(self.topdir, 'compose/Server/x86_64/os/Packages/spacer.rpm'))
+        helpers.touch(os.path.join(self.topdir, 'compose/Server/x86_64/os/Packages/x/pad.rpm'))
+
+        M = 1024 ** 2
+
+        with mock.patch('os.path.getsize', DummySize({'spacer': 4688465664, 'pad': 5 * M})):
+            data = createiso.split_iso(compose, 'x86_64', compose.variants['Server'])
+
+        self.assertEqual(len(data), 1)
+
+    def test_can_change_iso_size(self):
+        compose = helpers.DummyCompose(self.topdir, {'iso_size': '8G'})
+        helpers.touch(os.path.join(self.topdir, 'compose/Server/x86_64/os/.treeinfo'),
+                      TREEINFO)
+        helpers.touch(os.path.join(self.topdir, 'compose/Server/x86_64/os/Packages/spacer.rpm'))
+        helpers.touch(os.path.join(self.topdir, 'compose/Server/x86_64/os/Packages/x/pad.rpm'))
+
+        M = 1024 ** 2
+
+        with mock.patch('os.path.getsize', DummySize({'spacer': 4688465664, 'pad': 5 * M})):
+            data = createiso.split_iso(compose, 'x86_64', compose.variants['Server'])
+
+        self.assertEqual(len(data), 1)
+
 
 if __name__ == '__main__':
     unittest.main()