To know for certain, you need to:
1. Consult your owner's manual: This is the best source of information. It will specify the correct sizes for your specific year and model of E-250.
2. Use a Torx bit set: Purchase a Torx bit set that includes a range of sizes (e.g., T20-T60). This allows you to try different sizes until you find the correct fit for each bolt. Trying to guess and using the wrong size can strip the bolt head.
3. Check online forums: Search online forums specific to Ford E-250 owners for brake replacement guides. These often include pictures and specific tool requirements.
Never attempt brake work if you are unsure of your abilities. Brake failure is extremely dangerous. If you're not comfortable with the repair, take your vehicle to a qualified mechanic.