how to change video size in Final Cut Pro

The aspects ratio of your source material does not match the aspect ratio of your output device. That's not unprofessional, as you say it. Often it is just inevitable. Go watch a movie on any streaming platform and you surely will see black bars. Probably at the top and bottom, not the sides but it's the same problem.

Anyway, you have two options:

1. Crop the photos to reduce the height while keeping the same width until it matches the aspect ratio of your output device. If it's a computer monitor it will be probably 16:10 or 16:9. That means for each 16 pixel in width you need 10 or 9 pixel in height. I recommend to do it manually for each photo. Another option is to combine several photos in one mashup or panorama if applicable.

2. Get another output device that has the same aspect ratio as your photos. However, I think this will be much harder.

With some cameras you can set the aspect ratio beforehand, so for the future, there is the option to shoot with your target ratio.