There's no slide like you're thinking (sliding an entity down a line in a copy paste method). Very annoying since the copy/paste used can be very tricky to manipulate sometimes...once I had an item 20 feet up in the air but didn't notice it until I switched to side view. The way that I get around that is to draw a line directly to a spot where I want the new entity to start. For copying something like a wall this isn't necessary but if you want to place something like a bunch of tables: first lay out a grid (more accurate control of center-point placement), draw your table and put a handle on it (just a small line either on the x, y, or z axis), highlight the table and group everything (including the handle), copy it, then paste it down somewhere, while the lines are still blue grab the handle (at the very end of the line) and move it to where you want it and it should snap into position on your grid, ungroup the table and delete the handle, and lastly delete the grid when done. Lots of work, tedious work, annoying work. The effort is worth it in my opinion but I can be very anal like that when it comes to cad.
There should be a pop-up window when you hover over an entity or maybe right click on it (I forget, been a while); it pops up in the upper left of the screen. I always use it to make sure that my line is exactly as long as I want it to be (for castles I use 10 foot lines and for houses I use 8 foot lines...if I wanted to be really anal I would set the lines for 16 inches plus 1 3/4 inches -- which is how far the studs are apart in a modern wall plus the thickness of the 2x4). If you're drawing out a line there should be a small lil window on the bottom right of the screen that shows exactly how long it is as you're drawing it...you can type a number into that box and the line will jump to that length (just don't forget the ' for feet and the " for inches).