Assuming that you have selected the Rectangle tool properly…
Pick the start corner and drag out an approximation of the rectangle.
You can now do one of two things…
Enter the rectangle’s dimensions.
Do not click in the Measurements box, just type 1.23,4.56 and press enter.
Those values should then be applied to the new rectangle.
Note if your locale uses a comma as a decimal point you need to type 1,23;4,56
The values are then separated by a ;
Click a second point to approximate the rectangle.
Do not click on the Measurements box.
Immediately type in the exact dimensions required - e.g. 1.23,4.56 and press enter
[See note above about the .,; separators and your locale]
The rectangle will resize to suit.
Until you choose another tool you will remain in the current rectangle tool and you can continue to type in alternative dimensions, pressing enter each time to see the changes…
This change of its dimensions can also be used after initially making the shape with Option 1.