It doesn't matter what wattage resistors since HOPEFULLY!! you won't be drawing any current and therefore no heat to dissipate. I would construct the devider with a 150 ohm resistor in each of the AC leads with the 1 ohm across them at the end. Peraonslly, I would probably scale both the 150 and 1 ohm resistors up by a factor of ten, and therefore have 1k5 and 150ohm resistors. This would offer a little bit more protection if the end gets shorted to something - it will limit the current more.

You don't need to go into the mic preamp since you will be at line level - unless your mic preamp has a line input. Using that if it has one would protect your laptop more if things get nasty.

Good luck----and don't try to measure with this thing while in the bath tub......