For pie, you want the little sweet ones – 6-7 inches in diameter. The cooking is the same for all pumpkins (and winter squashes) though, it just takes longer.
Preheat oven to 350 degrees. Line a rimmed baking sheet with foil.
Cut the pumpkin in half vertically (through or right next to stem, or remove stem). Scoop out seeds and stringy stuff.
Place pumpkin halves cut side down. Cover with more foil, not tightly, more like a tent.
Bake until tender – until you can pierce the skin and flesh all the way through easily with a fork – about 1 ½ hours. Remove from oven and cool a bit. Scoop out flesh with a spoon. Puree using an immersion blender (easiest) or food processor, to desired smoothness. Pour off any excess liquid. Stores in the fridge for about a week.