No. Create an envelope template for each size. Word is not very good with
custom paper sizes, so it may be necessary to set a suitable larger size and
position the printed elements with the aid of margins. There is a sample
template on my web site for download, which demonstrates a few techniques.

Hi....
Its an simple issue you can resolve it by setting your
custome size.

Open your word document.
Go to Tools  select Envelops and labels
click on Envelops click on options
click on Envelop size there you have dropdown arrow
In drop down list you have custom size....when you click
on custom size you will get dialog box to set envelop
size ie., width and height then select your size click ok.

This will help you to use envelope of custom size.

When ever you create evelope check that in evelope size
you have custom size. click ok and proceed.

If above steps work out please reply to my mail address.
Thank you...
Have a Nice Day
bye bye.
with regards.
Wellwisher.