I'm thinking about making Ina Garten's Vegetable Pot Pie, but want to use a 9 x 13 casserole dish.

Since you're increasing the volume, it means increased baking time for the "thicker" vegetables, and so the dough might cook too soon.
If you have time, partially roast the vegetables on a baking sheet in the oven: toss them with olive oil, salt and pepper, roast them on a baking sheet for 20 minutes at 375 F. Then put them in your prepared dish and bake until top crust is brown (or whatever recipe calls for).
You could also top the 9x13 sheet using ready-made puff pastry.

You can definitely make this recipe in one dish as opposed to several small ramkins. I don't know how many people you need to feed with this, but I've found that her recipes are generous with the serving size, so tripling it seems like it would make a lot. The dough in her recipe makes four 8 inch circles, so you may end up with a lot of extra dough. If you do triple the recipe and bake in a 9 x 13, I might cook it for a little longer than the suggested time. Another good option if you are set on making a large amount is to divide into two 9 inch pie plates and you can always freeze one of them.