You're opening a big can of worms here. There's no simple answer to your question. A loudspeaker can handle many times its rated power for short durations and a fuse does not blow instantly either.
Let's make it simple, if possible: taking 100W for the woofer, using P = R.I², it suggests I = 3.5A.
The problem is that the rating of fuses is such that their nominal value is what they can withstand for an infinite duration. You want the fuse to blow before the speaker, right? So you start with a lower value, let's say half the calculated value, in that case, the nearest value is 1.6A, and you see how it goes; if the fuse blows even when using moderate power you need to increase its value. Or to use a slow-blow version of the same rating.
Yes, there are fast and slow fuses. I would start with fast ones because speakers tend to burn pretty fast too.
By using half the calculated value, you're limiting the long term power to 1/4 of the nominal power, which should be ok if you're listening to reasonably dynamic programme.
I don't know what's the rating of the tweeter, but they generally are between 10 and 20W, which would suggest a max current of 1 to 1.6 A, and the use of a 0.5 to 0.8A fuse. Again, you will ned to experiment. Tweeters are even quicker to blow, so definitely the fuse should be a fast type.
I've spent 20 years designing protection systems for high power loudspeakers (I'm talking about kilowatts here) and there is no system today that can guarantee complete safety and optimum sonic performance.
Finally, an attenuator doesn't change the amplifier's power, it changes its overall sensitivity, so putting a 3dB attenuator in front of a 200W amp expecting it to become a 100W amp doesn't work. It just means you need to crank the level 3 dB more to make it deliver its rated power.