to get the least harsh ride with springs, go with something like tanabe nf210. very modest drop, and gives ride that is very close to stock. any lower, ride will get harsher.
do a search in the faq for members example of wheel and tire size. 20" is the largest I would go with (IMHO); use of spacers will depend on your wheel offset and kind of look you are going for. do your research and make sure you know exactly what you want, pros/cons of wheel/tire size, etc, etc; worst decision you'll make is if you get a wheel/tire package that doesn't fit properly and give you rubbing issues.
2008 Lexus GS 450h - Obsidian/Black (fully loaded Cdn version) - For GSh mod list, click here
2010 Lexus RX 450h - Aurora White Pearl/Black - Package C (Ultra Premium Package)
JDM fog light junkie