1. There is no limit to increase / decrease volume / lun size
2. I did a research a while ago in regards of that issue for windows hosts, what i found is: there is no limit as long as you do not assign a drive letter. We currently use mount points on W2008 boxes and the average amount of luns 50+.
There is (or at least, was) limit to how far you can increase LUN size. This is due to requirement to preserve geometry information (actually, cylinder size); so when you grow LUN, it increases number of cylinders. It is limited to 65536, so it is how far you can grow it.