diff --git a/src/api/vdi.coffee b/src/api/vdi.coffee
index 7ed7afc62..461a59991 100644
--- a/src/api/vdi.coffee
+++ b/src/api/vdi.coffee
@@ -1,11 +1,10 @@
 # FIXME: rename to disk.*
 
 $isArray = require 'lodash.isarray'
-
-#---------------------------------------------------------------------
-
 {coroutine: $coroutine} = require 'bluebird'
 
+{parseSize} = require '../utils'
+
 #=====================================================================
 
 delete_ = $coroutine ({vdi}) ->
@@ -34,7 +33,7 @@ set = $coroutine (params) ->
 
   # Size.
   if 'size' of params
-    {size} = params
+    size = parseSize(params.size)
 
     if size < vdi.size
       @throw(
@@ -65,7 +64,7 @@ set.params = {
   name_description: { type: 'string', optional: true }
 
   # size of VDI
-  size: { type: 'integer', optional: true }
+  size: { type: ['integer', 'string'], optional: true }
 }
 
 set.resolve = {
diff --git a/src/utils.js b/src/utils.js
index dd22907d9..306ac998e 100644
--- a/src/utils.js
+++ b/src/utils.js
@@ -3,6 +3,7 @@ import forEach from 'lodash.foreach'
 import has from 'lodash.has'
 import humanFormat from 'human-format'
 import isArray from 'lodash.isarray'
+import isString from 'lodash.isstring'
 import multiKeyHashInt from 'multikey-hash'
 import xml2js from 'xml2js'
 import {promisify, method} from 'bluebird'
@@ -89,6 +90,10 @@ export const pFinally = (promise, cb) => {
 // -------------------------------------------------------------------
 
 export function parseSize (size) {
+  if (!isString(size)) {
+    return size
+  }
+
   let bytes = humanFormat.parse.raw(size, { scale: 'binary' })
   if (bytes.unit && bytes.unit !== 'B') {
     bytes = humanFormat.parse.raw(size)
diff --git a/src/utils.spec.js b/src/utils.spec.js
index f8463e150..a36216a8b 100644
--- a/src/utils.spec.js
+++ b/src/utils.spec.js
@@ -7,7 +7,8 @@ import expect from 'must'
 import {
   ensureArray,
   extractProperty,
-  formatXml
+  formatXml,
+  parseSize
 } from './utils'
 
 // ===================================================================
@@ -66,3 +67,25 @@ describe('formatXml()', function () {
 </foo>`)
   })
 })
+
+// -------------------------------------------------------------------
+
+describe('parseSize()', function () {
+  it('parses a human size', function () {
+    expect(parseSize('1G')).to.equal(1e9)
+  })
+
+  it('returns the parameter if already a number', function () {
+    expect(parseSize(1e6)).to.equal(1e6)
+  })
+
+  it('throws if the string cannot be parsed', function () {
+    expect(function () {
+      parseSize('foo')
+    }).to.throw()
+  })
+
+  it('supports the B unit as suffix', function () {
+    expect(parseSize('3MB')).to.equal(3e6)
+  })
+})