Hi Giles89,
Obviously everyone needs a couple of kites, and winter is a frustrating time for kiters in WA. You have a couple of options but first we need to understand the conditions you'll be looking at.
April - Lighter winds with plenty of days of 'off rather than 'on'. You could get a lot from a bigger kite in this month. With a 10m, a new line of thinking we have been promoting is if you are going to go bigger, go proper big. Skip the 12, 13 or even 15's, you only gain a couple of knots lower end with anything until you get up bigger than a 15. I'd suggest a 17! It sounds big, but it will work in your favour and you'll have more fun, more often.
May - Learn to surf, there is generally no wind at all in May. Glassy days with a bit of decent swell. You may get an Easterly down at Point Walter if you can get out of bed early enough, but don't delay as the winds drop to nothing by 10am usually. Pt Walter will be chaos with almost every school trying to complete lessons down there as well as every kiter in Perth going there. Crazy!
June - Not a lot of wind in June. Aim for the morning Pt Walter Sessions.
July - We sometimes get a few stormy sessions in late July. Wind is generally gusty and quite frontal. you need to get good at reading the weather that approaches from upwind. Watch for the grey clouds and rain squals. Use as small a kite as you can get away with, rig for the maximum wind speed, not the minimum.

August - Tends to be very frontal winds with strong squals and rainy gusts. This is not good wind for beginners and people with only one kite. A 7m is a good size for this kind of wind. You need to exercise extreme caution and never kite alone in these conditions.
September - Still get the odd storm winds and the beginnings of the first seabreezes. You need a big kite for this time of year, bust out the 17 again.
October - Lots of good 17m wind in October with seabreezes becoming more regular but often at around 12-14 knots.
In having said all that, I'd be looking for a bigger kite. Either try and pick up a 17m second hand or we have a few great LW kites in our store. Forget the 7m if this is your first winter. Getting used to a zippy 7m in frontal and gusty strong wind puts you squarely in the realms of dangerous kiting scenario's.
I reckon you'll advance faster, learn more and ride more often with a big kite rather than a small one. Look at a smaller kite in November!
I hope this helps.
DM