There aren't really any limits, within reason. However, for high value capacitors you will use a lot of area. For example 1uF could take up 500 square mm depending on the process. It is not a cost effective use of silicon, I would suggest. As Palmeiras said, you could easily make 1M ohm with a high resistance poly but that is by no means the maximum. You could make 10M, 100M - it is really just down to area.
At the lower end, again there are almost no limits, but there will be a minimum area with which to make a capacitor before you violate design rules. I have used 7fF capacitors in precision circuits and that was with a 0.8um process so I am sure 1fF would be no problem with smaller geometries. For resistance the problem is using a very low resistance. In both cases - low resistance and capacitance - the strays will be the limiting factor.
Keith.