Have a look at the data sheets and determine how much heat the chip is going to have to dissipate based on your measurements (i.e. use a DMM or similar device to determine the maximum wiper motor current). Next, see how many *C the device will heat for every watt of heat that has to be dissipated. Determine the maximum ambient temperature and the maximum temperature you want the TIP 147 to reach, now you know the allowable delta-T of heat rise. Then use a chart that the manufacturer of the TIP hopefully provides to see how big of a heat sink they recommend for a given Watt dissipation and allowable heat rise.
Googling brings up articles like this one at Digikey, this one at Sparkfun, or this one at Texas Instruments.