I don't think that I've ever heard somebody say they wish they made their reservoir smaller. There are both pro's and con's in reference to resevoir size. Larger reseviors will be more stable in temperture,
nutrients and Ph as it takes longer to heat/cool and there is more nutrients that can be depleated before causing a major problem. The PH also does not fluxuate as fast with the larger size, however is something goes wrong you have a lot of $$$$$ invested that you will have to get rid of. Also different plants need different nutrients and PH levels so it is not a one stop shop they can feed off of. Like plants will be able to utilize the same nutrients/PH but you will have to do your homework.
Do you have a picture of a system you are considering? That may be helpful to better answer some of your questions.
I also use well water and have not had any issues. I want to build a filter like GPS did but I have not yet. I guess I have been really lucky because I haven't had any problems due to well water issues, heat yes, Ph yes but nothing specific I can say "It was the unfiltered water" that caused any issues. I currently use 55 gallon blue barrels for my resevoirs but I plan to purchase the 175 gal totes with the cage next and partically bury them.