Alright, I put Bikemaster signals on my 08 600rr. They use a 12V 23W tiny bulb, one blew and I cant find any like that around my town. There are 12V 21W...would it be okay to use a smaller watt bulb? would it blow my fuse?

Also, my fuse did blow (the one for turn signals, brake light and horn), is it because I took the bulb out and leftthe empty socket then tried to use the turn signal?

It is very unlikely that because you left a bulb out you burnt a fuse, unless somehow water or some other type of conductor got in there and short circuited the socket.

There will be little difference in your amps when you're changing only a few watts on such a low voltage(less than 1/4 an amp). Power(watts)=Voltage*I(amps or current) so, if you take your 23watts at 12 volts -> 23=12I that is about 1.91 amps. And your new bulb the 21 watts is -_-> 21=12I that is about 1.75 amps. So less wattage = less amps. Although this only applies to a circuit in series, as when you add resistance and other factors in on a parallel circuit things become a bit more complicated. If you're uncomfortable running a 21watt bulb where a 23 was you can always buy an ammeter and see how many amps you are running across that fuse, if it is above 75-85% threshold, you can bump up the fuse 2-5 amps or so, but be careful how high you go as some of the wires wont stand up to very high amps(although im not sure how high you would need to go), as it is possible to melt the insulation.

I'm sorry if that was too long or drawn out for such a simple question. If you have any questions, I'd be happy to help even more.