try to use slider for disksize

app/UI/VmCreate/Sections/AdditionalDiskSection.php

@@ -33,8 +33,8 @@ class AdditionalDiskSection extends BaseSection implements ClientArea
         //size
         $field = new Range('additionalDiskSize', 5, $this->resourceManager->disk()->free());
         $field->addValidator(new DiskSizeValidator('disk',false));
+        $field->setDefaultValue(5);
         $field->replaceClasses(['lu-col-md-6']);
-        $field->setDescription('description');
         $this->addField($field);
         //bus
         if($this->configuration()->isQemu() && count($this->configuration()->getAdditionalDiskType()) > 1){