No higher than 60W, in fact I have 60W in both mine and I get an overshoot in the smaller fridge (both undercounter larder type jobbies), in retrospect a 40W probably would have been fine. Don't use a fan either, convection works for me.
I tape a washing up sponge to the side of the fv then insert the probe (ooh - err!) between the fv and the sponge. I think I read some experiment that showed that insulating the probe against the wall of the fv is the most successful way of maintaining a steady temp.
Likewise, though you can just wedge a sponge between the FV and the fridge wall, probe on the FV side. No need for any tape.