I don't remember seeing a house with a can bigger then 6" unless your count those old square things.
6" is just the standard size. 5" cans can use the R-30 65w also so there is not much differance except price. 4" can go 50w and same with the 3". Smaller cans can look nice but cost more and are harder to work with.
A taller cieling lets the light spread out farther. One rule of thumb is the light sperads out 1/2 the distance of the hight. In the back of some can catologs has a lot of details for different combos of cans, trims, bulbs, & hights. You can make 6" work for most things you just have to space them for what you need.
Someone could write a small book on lighting and different recomended spacing for what is needed. I would buy it. Otherwise it is lear by doing. Pay also attention to finished jobs that others have done. What works, what is shadowy, & find the goofs (like a can avove a frig).
Once you understand what is needed the next thing is making the customer understand. Too often they request too few cans to save. Then it can be a battle just to make them understand thy don't know what they need.
[This message has been edited by Active 1 (edited 06-03-2005).]