I'll try to answer your questions based on what I've noticed so far with my own heliostat.
1. The minimum resolution in large part depends on how far away your target is. The window I reflect the light into is about 50' away from the heliostat, and I think that 0.1 degree steps would actually probably be good enough for it. The sun moves 0.25 degrees per minute, and I have my heliostat update its position every 3 minutes. Even though it is 0.75 degrees off by the time it finally updates, it still stays on target fairly well. You can see this for yourself in the time lapse video I made on the heliostat projects homepage of this site.
So it's up to you how far you want to go with the resolution I guess. More certainly wouldn't hurt. The only disadvantage I can think of is that a higher resolution heliostat might be slow compared to a low resolution one. Not a big deal once everything is up and running since the sun itself isn't very fast, but it can be a pain when debugging if you have to wait a long time for the machine to reset itself.
2. From what I've seen with my heliostat, rigidity (or in my case the lack of it) is not too terribly detrimental to the heliostat's success. I can forcibly move mine along either the altitude or azimuth directions a degree or two, but it still ends up returning to its original position and reflecting the light to correct spot. Granted, that spot is only 50' away. Like before, it mostly depends on how far away you'd like to reflect the light. Make it better than I did obviously, but don't break the bank to do it.
3. I like the idea of several smaller machines too since they are more discrete. Something large would look odd (or more odd) if it sits in the front yard like mine does. Somewhere between 1-2 square meters might be about right?
I would make it as large as possible without being unwieldy, if that makes sense.
As far as keeping the costs down is concerned, I think it really just comes down to sitting down with some actual numbers and doing the calculations to see what size turns out to be the cheapest.
A few things that I have found out when building mine are:
Hook up wire for the stepper motors is one of the more expensive items (unless someone knows of a cheap source that I wasn't able to find), and the more heliostats you have the more you'll need of it.
Making the electronics and wiring everything together for each heliostat is very time consuming.
Aligning each heliostat and programming the targets is also very time consuming. I have ideas for making this task go faster by adding features to the Arduino program, but either way it will still take time out of the day.
Like you said, I would start small and first see if you can just get everything to work right.