How much should I alter the measurements of this recipe if I use a tinfoil roasting pan that measures 13 3/8” x 9 5/8”? Is it

Katheryn K (recipe author) answered on foil pan.
And size is very similar to original.
For more or future help on converting recipes to other pans (either what to have, or to change size of recipe) see charts like this one from allrecipes. Other sites, like King Arthur Flour, also have them.
http://dish.allrecipes.com/cake-pan-size-conversions/
And here's an article of similar advice from boulangere extraordinaire Alice Medrich.
https://food52.com/blog/13239-how-to-make-your-baking-recipe-fit-your-pan-size

Hi! It doesn't matter what type of dish you use and and I don't think a slightly different sized dish will make a difference either. But if you are concerned add a little extra potato, cream and cheese and seasoning, it's a pretty forgiving recipe.