The size of a marble countertop frame for a typical desktop can vary depending on the specific requirements and dimensions of the desktop. However, there are some standard sizes and guidelines that can help you determine a suitable size for a marble countertop frame. Here are some considerations:

1. **Standard Desk Sizes:** Typical desk sizes can range from small writing desks with dimensions of around 36 inches (91 cm) in width and 20 inches (51 cm) in depth to larger executive desks that can measure 60 inches (152 cm) or more in width. Consider the width and depth of your desktop when choosing the frame size.

2. **Overhang:** A common design for desks includes a slight overhang of the countertop or desktop on the frame or legs. This overhang is usually in the range of 1 to 2 inches (2.5 to 5 cm) on the front and sides of the desk. Make sure to account for this overhang when determining the frame size.

3. **Leg Support:** Consider the design of the desk's leg support system. If your desk has four separate legs or supports, the frame size should accommodate the distance between these supports. If you have a single central support or pedestal, ensure that the frame fits within its footprint.

4. **Desk Functionality:** Think about the intended use of the desk. If it's primarily for a computer workstation, you'll need enough space for the computer monitor, keyboard, and other peripherals. If it's a writing desk or work surface, the frame size should provide ample space for tasks.

5. **Aesthetics:** Consider the aesthetic balance of the frame with the desktop. The frame should complement the overall design and style of the desk. The frame's size and shape can impact the desk's appearance.

6. **Personal Comfort:** Ensure that the frame size allows for comfortable legroom and ergonomic use of the desk. The frame should not obstruct leg movement or create discomfort for the user.

7. **Customization:** If you have specific size requirements or a custom-designed desk, you may need to have the marble countertop frame custom-made to fit your exact specifications.

8. **Marble Slab Size:** Keep in mind the size of the marble slab or piece you intend to use for the countertop. The frame should be designed to accommodate the dimensions of the marble slab without overhanging or leaving gaps.

Typically, a suitable frame size for a standard desktop might fall within the range of 36 to 60 inches (91 to 152 cm) in width and 20 to 30 inches (51 to 76 cm) in depth. However, it's essential to measure your specific desktop and consider the factors mentioned above to determine the best frame size for your marble countertop. Customization may be necessary to achieve a perfect fit and desired aesthetic for your desk.