What version of SU are you using?
“Free plan” could mean the Free web version you open in your browser.
But it might also mean you are using the trial period of SU Pro or Studio.
As you have found, you cannot change the size of an object in SketchUp by trying to alter the dimension value.
You can only do that by editing the drawing itself.
Depending on which version of SU you are using, web, iPad or desktop, the appearance of the tools you can use to edit the drawing will differ. And depending on what it is you are trying to edit, the approach may be different too, requiring different tools.
Upload your model to the forum here so we can see what you are trying to change. If the file size is more than 16Mb, upload it to a file sharing service like Dropbox, OneDrive, or WeTransfer and post a link here. Make the model shareable and editable by others.
And correct your profile - to do so, see How to update user profile - #2 by DaveR
And as noted in an earlier response, watch the tutorial videos at http://learn.sketchup.com, and also on the Sketchup Youtube channel.