Hi Charlie.
Thanks for your enquiry.
The smallest are the "hand turned" organs, which are usually operated on a small trolley, so they would definately fit into your venue.
Then the size increases in stages all the way up to the very large Fairground Organs, so there should be something larger than a hand turned which would fit through your arch. The problem is that organs tend to be mounted in a trailer, so that is where the size is limited.
I converted your measurements into feet and inches for us old-fashioned folk! Height: 5' 4", Width: 4' 9".
I will have a look on the Organ Availability Register in the South East region for one that may fit:
http://www.fops.org/oar/search.php?regi ... uth%20EastWhere is your party to be held and what is the date?