It is thus.
Activate the Rectangle tool.
Pick a point to set one corner of the shape.
Now stretch out some arbitrary rectangle in the direction you want.
Tip: it’s a good idea to make it elongated so that you can see which SketchUp thinks are the two dimensions.
Click to make the temporary rectangle.
At this point the Measurement box [aka VCB] will show the two dimensions, and from that you can deduce which you want to take as the first and second values.
Without doing anything else you remain within the rectangle tool.
Now type in the desired values and press ‘enter’***
The rectangle should resize to those exact dimensions.
As explained in earlier posts an existing rectangle can be adjusted too.
***Note that you do NOT click on the Measurement box - just type the values and they’ll appear in it.
Also note that how you type in dimension values depends on your ‘locale’.
In the USA, UK and much of the Commonwealth the decimal separator is a . and the list separator [to separate the two values] is a ,
However, in most of the EU and ‘latin’ countries the decimal separator is a , and then the list separator becomes a ;