I see Johnny's point here, infact i see both. But here's my 18 pence worth for what it is.
So your 5'4'' and 9 stone. Thats pretty light. But here goes.
Minimau - 7'6'' - 7'9'', 20 1/2'' - 21'' Width - Thickness, well talk to someone in the board shop about that.
But i reckon at your weight a minimau 7'6'' - 7'9'' should give you enough stability. But as Johnny said, don't worry about shortboards yet. I was a fool and struggled on my custom board all the way from Oz for 3 yrs!!
I used to ride a 6'6'', round tail Greg Webber Pro-Model. It was like being a novice jockey trying to control a race horse who really wanted to gallop fast! I spent most of my time swimming, so rarely managed to stand up. So a minimau is what you want, then you can go either way when you have the ability to do so.
Think of it this way, What would you rather do?
Have a custom made shortboard, look like the other really good surfers, but look like a benny when you can't do anything but fall off your toothpick?
Or, walk down the beach with a minimau, get in the water, catch more waves with less effort and manage to stand up, then be able to practice carving and turning from the drop in, and have a good time and people see that your having a good time?
Which sounds better to you?