try to use slider for disksize

app/UI/VmCreate/Sections/AdditionalDiskSection.php

@@ -28,7 +28,7 @@ class AdditionalDiskSection extends BaseSection implements ClientArea
     public function initFields()
     {
         //size
-        $field = new Text('additionalDiskSize');
+        $field = new Range('additionalDiskSize', 5, $this->resourceManager->disk()->free());
         $field->addValidator(new DiskSizeValidator('disk',false));
         $field->replaceClasses(['lu-col-md-6']);
         $this->addField($field);

templates/client/default/ui/core/default/widget/forms/fields/range.tpl

@@ -38,7 +38,7 @@
            max="{$rawObject->getMaxValue()}"
             {if $rawObject->getName() == "memory"}
                 step="512"
-            {elseif $rawObject->getName() == "disk"}
+            {elseif $rawObject->getName() == "disk" || $rawObject->getName() == "additionalDiskSize"}
                 step="5"
             {/if}
             {if $rawObject->isOutputVisible()}