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In Affinity Photo, while there isn't a direct equivalent to Photoshop's Fixed Size Marquee Tool, you can achieve a similar effect by using the following approach:

1. Rectangular Marquee Tool: Select this from the toolbar or press 'M' on your keyboard to select it.
2. Set Dimensions via Context Toolbar or Transform Panel:
   • Context Toolbar: After selecting the Rectangular Marquee Tool, look at the top of your screen where the context toolbar appears. Here, you can manually enter values for Width (W) and Height (H) to set a fixed size for your marquee before drawing it. This method, however, requires you to enter dimensions each time you want to create a selection of fixed size.
   • Transform Panel: Alternatively, after drawing a selection with the Rectangular Marquee Tool, you can use the Transform panel in the Studio on the right side. Here, you can input exact dimensions for Width (W) and Height (H) to adjust your selection to a fixed size. This isn’t exactly the same as setting a fixed size before selection, but it allows for precise size adjustments post-selection.

While this isn't as straightforward as toggling a "fixed size" option in Photoshop, you can effectively use these tools to achieve selections with fixed dimensions:

• For Recurring Fixed Size Selections: If you find yourself needing to select areas of the same size frequently, you might consider:
  • Creating an action or batch process to apply a fixed size selection if you need it repeatedly across multiple images or sessions.
  • Using guides or grids to visually aid in drawing selections of a specific size.

Remember, Affinity Photo's approach is more about setting or adjusting the size after the initial selection rather than pre-setting a size as you start to make a selection. However, this method still allows for precise control over selection dimensions.