The no partial construction is not a problem and not anything new. To me that would affect my decision to build the 2000 man army or the smaller quicker 200 man army. Either way I just can't see it as a problem that you don't already face now. Do I build the 1550kT Baseship or the 350kT Destroyer? Maybe that's not the best of examples but do you see my point? Don't build the large powerful 2000man unit if you know you are about to get invaded.
The scale on map can be manipulated by changing the picture called on or in the vehicle size file. If I am not mistaking it should be this line: "Maximum Model Size".
You are right that one person could use many smaller units to try to tie up a larger unit. What you are forgetting about though is that the smaller units (by the way there wouldn't be any single units) have less weapons and armor. So 20 smaller units each with 10 weapons equals 200 shots at a single large unit with 2000 weapons. Your small units will get chewed up pretty quick. You will need to invade with something that has a bit more endurance and fire power!
Keep in mind in SE5 the program decides; based on the strategy, how many weapons to fire at each attacking unit. Put another way I may need to fire 20 shots to knock out one of your small units. 20 shots times 20 (from the above example) is 400 shots. If I have 2000 shots (again from the above example) that leaves 1600 shots. Still think you can tie my unit down?