I found this thread from 2016, but the issue still seems to be the same.
This is wildly intuitive.
I want to be able to select an object, such as an arbitrary rectangle, and then type the dimensions I want it to be some where, such as 200cm x 8cm x 4cm.
The suggestion of typing it in the lower right dimensions does not work it would seem. Is this a subscription feature only?
Start drawing the shape with a click and release and move in the rough direction you want to go, then let go of the mouse and type in mid air 200mm,800mm and hit enter. and you will have a rectangle that size.
Do Not attempt to click in the box bottom right, it is only for display.
I did that and nothing happened, the box did not change size, yet I saw the dimensions were being typed in the lower right.
and did you hit enter?
yes :-\
Youāre in Germany I take it, try with ;
ahh ok i was trying to use X to seperate values, but the comma seemed to work now, danke! 
is it not possible to change dimensions after the object has been defined? Or is the move tool necessary in that case?
You can keep typing in new things and it will change until you have made another action.
You can use the move tool or the scale tool to change things.
Going forward, use the exact same syntax as is already displayed in the Measurements window. As you drag out the rectangle you will see the separator between the two dimensions. When you perform other operations which involve the Measurements window, look at what is displayed before you type.
The Move tool is certainly one good way to adjust existing geometry. The Move tool accepts numeric input in the same fashion as the Rectangle tool, etc. In other words, if you want to move something (such as one edge of a rectangle) by 113.6mm, you can do that exactly, by activating the Move tool, clicking (and releasing) on the edge you want to move, moving the cursor (and selected edge) in the direction you want, then letting go of the mouse and typing a dimension and pressing Enter.
You can also do it using the scale tool, the difference with the Scale tool you can either scale by a percentage or to an exact size by using the units.
So a cube can be scaled in 1, 2 or 3 dimensions to exact sizes as long as you use the unit when typing, otherwise it will default to a percentage.
Thank you for explaining this. OMG. This UX violates at least 3 of the 10 core principles of human-friendly UI design. I literally never, ever would have guessed that this is how to manually set dimensions ā and Iām a professional UX designer & work with many different types of graphics and layout packages, from Powerpoint to Adobe to Figma. The difficulty setting dimensions in Sketchup has been driving me bananas since it was originally acquired by Google & I started using it.
Having to come to a helpfile for something so basic is such a red flag.
Goes to show what one does not know.
When using the scale tool it tells you in the status bar that you can add a scale factor or dimension and the Instructor Tray explains it quite clearly. Surely you as a professional UX designer are familiar with people not bothering to learn how to use the tools you design.
No, Iām not, because the software I design and manage doesnāt make basic mistakes like this. I thanked you for explaining it ā no idea why you decided to take it personally. Something as basic as numerically scaling a graphical object in a design package should not require going to the documentation. Full stop. Many of the other interactions in Sketchup are fantastic and itās crazy how a team that so capably designed those intutitive interactions made this decision and has stuck with it for 15++ years.
Have a nice life & thanks again for explaining it above.
But it is clearly explained right there on the screen, twice, no need to go to any help file.
Iām not taking it personally, this is just my opinions and curiousness.
Iām not a professional (or at all) UX designer. Can you please list these principles!
Iām really curious, and do not take it personal: You, as a professional how would design this tool, considering to āfit toā the general Sketchup UX?
As Box and RLGL pointed out, you do not need to guess. Just read the statustext or leave open the Instructor window until you learn how to use the specific tool.
Ignoring the instructions is a bad reason to say you couldnāt figure out how to use it.
In my opinion, the UX for this is āfitā to general UX of SU.
Specifically related to the Scale tool I agree, it is not perfect, because you can not enter the 3 dimensions if you are using e.g. Hungarian keyboard layout and regional setting. (There is a workaround for thatā¦)
But this is not related to my general opinion:
It is not at all a shame - but useful - for someone to read the instructions to use the tool in question.
Where can your work be seen?