In Photoshop, you are able to choose the selection type for the Marquee Tool. Selection types in Photoshop are, Normal, Fixed Ratio and Fixed Size.
In Affinity Photo (v1.6.4) there is no selection type for the Marquee Tool. However, you are still able to select using Fixed Size.
Fixed Sized Selection in Affinity Photo
Choose the ‘Rectangular Marquee Tool’ from tools, or press M .
Select any area on the Canvas
Change the size of the selection from the Transform panel. Size adjustment is marked W: for width and H: for height.
Drag the newly sized selection to where you want it.
Fixed Ratio Selection in Affinity Photo
The Fixed Ratio Selection in Affinity Photo is missing in the Marquee Tools. Unfortunately, Fixed Ratio Selection is only available in the Crop Tool, which can only be used for cropping
Choose the ‘Crop Tool’ from tools, or press C .
Choose ‘Custom Ratio’ from the Mode: drop down menu
Change the values for the custom ratio
Click and drag the crop selection on to the canvas