How do I double this frittata recipe to serve 8?

Having not done it yet myself, I think it absolutely can be doubled. You will know that the frittata is ready when it is puffy and burnished golden, with a bit of "crust" around the edges. I agree with Antonia to be careful about the pyrex - the ingredients won't be totally cold (the roasted elements will be warm or room temp, for example), but it's a good thing to be mindful of. A metal roasting pan, or a gratin dish, are an easy "yes"!

If you have an oval gratin dish or similar baking dish, you could test to see what volume it holds and use that. I made this recipe as drafted in a rather shallow vintage Le Creuset enameled cast iron gratin dish and it turned out beautifully.

I am confident that a double batch would work just as well in a larger oval baking dish. Just make sure that it is not too deep, as that will affect the cook time.

Also, I'd probably avoid using Pyrex-type dish, because if you pou cold ingredients into the heated dish, it will shatter. I'd opt for porcelain, ceramic, enameled cast iron, any other shatter-proof option. ;o)

From a look at the ingredient list, the volume of the original recipe is just over
5c and double that would be estimated 11 cups
But a 9x13 pan holds 14 cups, so the double frittata might be a bit thin in that size pan, and cook up drier than normal.
Options to help you include mixing up the raw batter in a bowl whose capacity you know, say 3 quarts, and see how it measures up.
And/or use one of the many baking pan volume charts to find a pan that can hold 11c capacity.
https://www.joyofbaking.com/PanSizes.html
Possible choices, for example, a 10" Bundt or spring form pan, a 9" tube pan, 10" round or square baking pan.