Working at a selfstorage site and looking for a widget to calculate how big the space must be. For instance inventory kitchen 5 m3
This looks like a very custom requirement. I don't think there is a widget for your purpose unless someone can create it for you. See Create and share a custom widget for Adobe Muse | Creative droplets for info on creating custom widgets.
Just to be sure, you can also have a look at some of the widgets here to see if any of them meet your purpose.