Sure, a metal large-size first aid box with a lock is typically used for storing medical supplies securely. These boxes are often made of durable metal such as steel, and they come in various sizes to accommodate different quantities of supplies. Here are some features and considerations for such a first aid box:

1. Size: Ensure it’s large enough to hold all your first aid supplies comfortably. Dimensions might vary, but a larger size is generally more versatile.
2. Material: Opt for sturdy metal like steel for durability and protection of contents.
3. Lock: Look for a box that includes a secure locking mechanism to prevent unauthorized access. Some options include key locks or combination locks.
4. Interior: Check if the interior has compartments or dividers to organize different types of supplies. This helps in easy access during emergencies.
5. Portability: Consider if the box has handles or is lightweight enough for easy transportation when needed.
6. Visibility: A clear window or labeling area on the outside can help identify the box as a first aid kit quickly.
7. Regulations: Depending on where you intend to use it (like workplaces or vehicles), ensure it meets any specific regulatory requirements for first aid kit storage.

You can find such first aid boxes in medical supply stores, online retailers, or even hardware stores that carry safety equipment. Make sure to stock it with essential first aid items such as bandages, antiseptics, gloves, and medications appropriate for your needs.