Which comes first depends on a variety of things that are hard to define, easiest thing to do is look at the measurement box as you draw out your rectangle.
Try exaggerating the shape, pull it out long and thin, look at the measurement box for which is larger and what symbol is used between the numbers. Then just let go of the mouse, and without trying to click in the box, just type the sizes you want in the relevant order separated by the , or ; depending on which is shown and hit enter.
@Skareby, start drawing the rectangle by the first corner. Then the lower right screen corner āMeasurementsā field should indicate you which one comes first. There are different conditions that determine which comes first. (Rectangle is free or attatched to other geometry, which plane etc.)
I assume you are drawing a rectangle in a fresh drawing. When you start drawing the rectangle, SU will assume you want it on the āground planeā (ie. Z=0).
When you type in dimensions, SU will assume the first dimension is in the X direction, parallel with the red axis. The second will be the Y dimension, parallel to the green axis.
Not always. Itās variable. I think it depends (perhaps among other things) on which direction you move the mouse most, and puts the longer dimension first - but the advice above is the best guide - make the original mouse move such that one direction is obviously bigger than the other and see whether that is x or y (or other axes if you arenāt in the x-y plane). And use comma or semicolon separator as shown in the measurements box.