Hey all. 

I am looking for the option to fix nodes. I have two objects that are aligned at one common node. I want to fix them so that if i change the measurements of one of the objects, these two nodes stay where they are.

Any idea?

Afaik there´s no such option. You can exclude points from transforming - but shape will alter like here:

Oh that's actually what i need, thanks! In my case i have a rectangular and want to change the size, but two points should not move 

Actually, not really. I tried it but it not always does what I want.

On the other hand: Is it possible to change the size of an object and set in which direction it changes? If I increase the width for example, it always increases to the right. Can I change that?

Maybe like so. Check out the faint lines indicator:

https://wiki.inkscape.org/wiki/index.php?title=Release_notes/1.2#Selector_Tool

The Transform panel allows numeric input and will scale from the center of the bounding box.

Thanks. But I need one of the sides to not move when scaling. If I enter a new measure in the top, it extends to the right. If I use the Transform panel, it scales from the center. I actually need left :-D

Same procedure:

I understand, but I want to enter fixed and precise values, not by hand

I found my solution. I just add a ruler at the current position of one of the rectangular sides. Then I can double click on the ruler and move it relatively, e.g. 2mm. And then just snap all nodes to the new ruler position