You'll need to check the datasheet for the resistors.

The simplist option is to treat the load at switch on as a pure short circuit, which then gradually reduces to normal load.
So assuming, 240V, with a 100R resistor, you're going to have a 2.4A flow producing 576W of heat.
At 150R, current and power is reduced to 1.6A and 384W.
These loads should only remain for under a second, so you need to look at the datasheet and see what the surge/intermittent max dissipation is for each resistor.
The 5 and 10W is the continuous disipation rating, but they will have a much higher intermittent rating, typically given as 5, 10 and 60second intervals.

Also keep on eye out for any maximum current ratings, otherwise your resistor will pop like a fuse before it even warms up.