Ideally, you would get a DC power adapter for your laptop, rather than converting from DC to AC, then back to DC again. But if you want to be able to plug in other devices, you should get an inverter that can handle at least 90W.
You could get the 60W adapter and try it, but you should first measure the AC adapter's draw under varying loads while plugged into the wall--for example, maxing out the screen's brightness and ripping a CD or DVD while the laptop is fully charged, vs. doing the same while the laptop is charging.
If you do end up getting the 60W inverter and you happen to overload it, the fuse in the inverter should blow. If the battery is already charged when you hook up the laptop, you might be fine. But if the battery is partly depleted and the AC adapter tries to charge it, you'll probably end up drawing more than 60W. A slightly annoying workaround would be to physically remove the battery when using the laptop in your car.