How do I ensure that pita forms a nice big, even pocket when baking?
The magic word is "water" - the dough needs sufficient moisture to stay moist while it expands to one big pocket and enough to generate the steam to make that pocket.

Most people when doing doughs make them too dry because they're easier to work with.  As the dough rests, it will pull together more.  Ideally make your doughs for this kind of bread slightly wetter than you think and then check it after its been mixed for 30 minutes or so.  Additionally, as you knead and then in the fermentation stage it will firm up some.  Difference in water content of the dough causes many of the consistencies for bakers.  It will also depend on how much it dries after its been rolled so thin - keep it covered.

Alternatively, if you've put your first one in and found out its too dry, then try spritzing the top with a little water and let it rest there for 5 minutes or so. 

I tested batches with spritzing and without, and so long as there's enough water in your dough and the oven is hot enough to make the steam - it doesn't seem to matter. Every one came out perfect.