Koviac,
To start:
1) Always prepare your work for layouts in the View Map of the navigator.
You show a screen capture from the project map in your first post. The project map is a very bad choice in terms of preparing your work for the layout sheets.
2) Do not place details or any figures directly on stories. Stories are strictly for placing the elements required to build the 3D model. Walls, meshes, objects, etc. Again I must emphasize we are looking at your project map, not a place for work of any kind. There are exceptions of course, but for now follow this golden rule: do ALL of your work in saved views.
Let me digress and explain what the Project Map (PM)is. PM is the digital foundation of the model, GS calls them "View Points" a very confusing term, because they are not "views" at all. The PM is a "dumb" storage bin for the raw data of: walls, objects, etc. It is the visual data base of the model. In PM you CAN NOT record any visual changes you make (pen settings, scale etc). On the other hand the View Map is a place where you can change how you see the raw information of the model AND RECORD THOSE CHANGES. This slight difference is the difference between being confident in AC and being constantly confused.
OK back on track.
3) place your figures, external details etc on worksheets. Then move your focus to the View Map make any changes to the pen sets, scale, etc in View Map and save those changes (you will need to access the help files for more details). Once you have done that, only then do you place the view onto the layout. Again AC has a weird ability to confuse. NEVER right click in a view window and use the pet palette to "save and place on layout". ALWAYS hover over the saved view name in the Navigator View Map, click it and choose to "place on layout". Believe me, this small step makes a big difference.
Now let's finally get to your layout sheet and scale issue. Once you have done the above. you can right click on the image you see in the layout, and via its pet palette, change its scale to fit your needs. If the original image was a PDF, you may have to first resize that image while in the worksheet itself to get to scale. Then in the layout book you will be able to change the scale via percentages or by referring to other standard scales.
HTH,
Mark