Should I size up in an insulated jacket to fit more layers underneath?

Generally no. Insulated jackets are designed to be worn with a base layer or thin mid-layer. Sizing up can create air pockets that reduce warmth and make the jacket feel baggy. If you need to wear thick mid-layers, you likely need a warmer insulated jacket rather than a larger size. The exception is if you're between sizes—in that case, sizing up is usually better to avoid compressing the insulation, which reduces its effectiveness. Always try on with your intended base layer.