Unfortunately this is currently not supported in the IWWebPlanner control.
You need to take in account that when you want to display 12 hours with a resolution of 1 minute, the absolute minimum height will be 12*60 = 720 pixels and then there is not even the timeslot border size taken in consideration. Am I right to understand this is what you want?
Maybe I can convince my client that 5 minutes will do also. But yes, it would be very nice if the scale on the axis is indepentend of the display-resolution. (sorry for my bad english)..