Hi Windywishing,
There are no retail shops in SA anymore but there are a couple of people who are based in Adelaide that you can talk to about new gear.
Ty Andrews from Ocean Surf Co -
www.facebook.com/oceansurf.co/PH: 0418826459
Jack Fletcher from Fletcher Boardsports -
www.facebook.com/fletcherboardsports/PH: 0418807141
Both are great guys who will be very helpful.
In regards to board size, it will depend on your past sailing experience, ie can you beach start and waterstart, where do you plan to sail, what do you want to achieve?
Cheers
Bondy