You know what we're going to say: talk to a professional. The most reliable estimates of what equipment you'll need come from people who've already installed dozens of renewable energy systems in your area. However, we know you're going to try to figure it out for yourself, so here are some guidelines!
There are three main ways of figuring out how much PV space you'll need: daily use, net-zero and a simplified guesstimate.
Daily Use
This is the more common approach. It estimates how much electrical energy your home uses and calculates how many PV panels you'll need to generate enough power to avoid blackouts in the middle of your favorite soap.
The first step is to go around your house and make a list of every single appliance, gadget and other plug-load. It'll be a long list for most homes. Don't forget to include ghost loads as well, for anything you leave on standby (and remind yourself to unplug non-essentials at the socket).
While you're making that list, take a look at the info sticker for each item and note its power consumption in Watts. If you can't find the information, look it up online when you've finished your list: most manufacturers provide details and, if not, you can ask on forums and green power sites.
Next, you need to estimate how many hours per day each appliance or other electrical device is used. You'll probably want to list hot and cold season usage for each item, too, since it will differ.
Now you need to multiply the values for power use and number of hours to work out how much power each item uses. Keep the figures for the two seasons separate! This is the total amount of power you must generate daily from your PV installation to cover all your electrical needs.
You also need to figure out which season will need the larger number of PV panels. To do this, divide the total daily power use by the insolation value for the season: this tells you how many kWh your system must provide. You may use more power in summer but, since there's more sunlight, you might need fewer panels.
Finally, once you know which season requires more output, calculate how many panels you'll need by dividing the power output by the output per panel. You can calculate the space needed by multiplying the number of panels by their individual size.
Net-Zero
The calculations for a net-zero effect are very similar to those for daily use (which is really peak use, if you think about it). The main difference is that you can take the year as a whole.
Go through the same steps as for daily use, above, but take an average of the two seasons' electricity usage. Take an average for insolation for the whole year and use the two averages to work out your PV panel needs. Over the entire 12-month period, your over- and under-production of off-grid power should balance out to give you the net-zero effect you desire.