The medium mini,

http://www.windycityparrot.com/Merchant2/merchant.mvc?Screen=PROD&Product_Code=91401071002&Category_Code=MACAWL

or the Medium Long

http://www.windycityparrot.com/Merchant2/merchant.mvc?Screen=PROD&Product_Code=91401071001

will be fine for your Galah, the large is really only needed for macaws and large cockatoos, although going larger is okay too. Just do not go small for a galah.

Thank you