You can also use the scale tool to change your rectangles. If you select the whole rectangle and then invoke the scale tool, you can pick which handles you need(the handles are the little squares that appear) type in the overall dimension you want for that side, then type in the units after the number and hit enter. If you don't type in the units after the number it will be scaled by a ratio and not a dimension. I work in metric so I would type something like "340mm" , if I typed "340" it would scale by a factor of x340.
Also for inferencing, when you want to move something you can lock it to move on a particular axis by pressing one of the arrow keys after selecting the object with the move tool. Or if you start it moving on an axis hold the shift key and it will be locked to that axis. The inference engine can be finicky to get used to but it is one of the most powerful aspects of SU tht make it so fast and easy to use. It helps if you orientate your view so it would be more likely to move on a given axis.
Download the user guide from the SU website, even if its an older PDF version it is still mostly relevant. Be patient and try some tutorials and search this forum, someone else has usually already asked the same questions that you are likely to. Good luck and persevere, its worth it.