Pretty harsh, Mitch, especially when you put it in the first line of your answer!
Since Michael self identified as a “real beginner”, he may not even know what a parametric modelling program is. In my reading of his post I assumed he was having difficulty finding how to do these things at all.
So, Michael, the real answer is in Mitch’s 3rd paragraph. For changing the size of an already drawn object, use the scale command. You can scale in 1 dimension, 2 dimensions, or 3. Since it’s a scale command, you don’t just tell the object what the new dimension should be; instead you tell it a factor by which the existing dimension should be multiplied.
Have you become familiar with the command help? There’s plenty of detailed info in it about how to do things in Rhino. It may take several readings and some trial and error, though, to wrap your mind around the basic concepts of how things are done in 3D modeling in general and Rhino specifically. Your scientific programming expertise may actually hinder you a bit until you get over the initial hump, but will probably ultimately help your understanding of what Rhino is doing “under the hood”.