What size roasting pan should I use for 6lb brisket? What about an 8lb brisket? Do you recommend a brand or kind?

Go for as big as your oven allows, with some space all around for air to circulate. This allows various sizes & shapes of roast, plus veg. And as sturdy as you can afford...so meat doesn't burn or pan buckle when you remove it. Some like enameled cast iron, which is sturdy, maintains even heat & looks beautiful. Also costly & heavy even before you add the meat. I prefer uncoated cast iron, stainless steel, or even enameled ones that came with old ovens. In a pinch, or before you decide, use a disposable foil pan with a baking sheet under it.