I created a default sized page. Added a second page using the "pages tool". Then I dragged it to resize, now it's in custom size. Then I wanted to set the precise width and height to 1500 pixels and 1000 pixels. 

So I selected second page and tried two different ways. 

First I tried to set width and height in export menu, which doesn't have an "unlink" button, so when width is changed, the height is changing, which I didn't want to happen. (it is expected as it is an export menu, but still I tried)

Second, I tried to change the size through the document properties. As you can see, although the second page is selected, document properties only make changes for the first default page.

I don't understand the use of that feature as the first page also a page and the document properties only applies to the first page, even though I selected the second page. The software acts like "Ah, I'm blind I can't see a second page, so I'll just apply any settings to the first default page" 

Is there any way to custom set (to type and set the precise size) after I already in the custom set status. And I don't want to create default template as I need to change the size precisely on the go as needed. 

This is a solid basic feature, so there must be a way right? 

Using page tool, if you use the first icon of the tool control bar (a page with a + inside), you create a second page of the same size as the first one).

Next to this icon, there is a textfield where you can manually resize the selected page by entering its new size.