[How can I downsize a picture proportionally?]

This is not what I mean. 

 

I do have a picture. I want to cut the picture by using the sniping tool as shown on my upload. But if I use the sniping tool manually, the proportion of the picture will get lost if I am downsizing the picture 9x4 cm for example. So let me put it this way: I want to select a certain area of the picture and cut out this area. But the selection process should keep the proportion of the picture. 

 

[How can I downsize a picture proportionally?]

Hi, I am a new user of Affinity Photo and searched i a lot of tutorials. Maybe someone can help me:

 

I do have a picture. I am going to select a part of the picture by using the sniping tool. But how can I use the sniping tool that the proportion of the picture will stay the same? 

Is anybody having a hint where to find a tutorial video which is relevant to this topic? 

 

Thank you.