Sorry to necro an old post, but the google results for "trimming transparent pixels in Affinity Designer" has this as the top result.
If you own both Designer and Photo, you can click on "File" in the upper left and about 2/3 of the way down, select "Edit in Photo". Once Photo opens, you can then click "Document", and again about 2/3 of the way down select "Clip Canvas". Once that is done, you can then click "File" in Photo, and 2/3 of the way down, select "Edit in Designer". Once the image disappears from Photo, you can close Photo, and resume your work in Designer.
It's sounds worse than it actually is. Hope that helps!
- Sven2157