I am getting ready to do an ironbutt fundraiser ride (48 States in 10 Days for child abuse prevention). I was like to gave a flag behind me that states what the ride is about. Big enough to be legible to passersby, but so large as to create a hazard. What is that optimal size?

 

3x5 as long as you have a good enough mount.

 

The 3X5 flags will self destruct ! I use the smaller flags on my antennas. I think the size is around 12"X16" or close. They have a sleeve that goes over the antennas and I then put a small hole at the top of the sleeve and put a small wire tie to fasten it to the antennas.
We never do missions at highway speeds with flags out. Nothing over 40MPH. I use a flag sleeve to cover the flags that are rolled around the pole. Easy on/off.
You wouldn't believe what happens if a big gust of wind hits you with 3X5 flags out. I pull 3 3X5 flags on missions. One US flag, one service flag, and a PGR flag. If no service flag there is a POW-MIA flag there.
Your bike , you could put the smaller flag mounts on a rail. Put a hitch on for a 3X5 in towns, or a front of bag mount for 3X5's. Do you put on a tour pac when you travel? A lot of guys do the small flag mounts on the tour pac rack.
Just some thoughts.........

 

 

I have ridden with a 3x5 flag many times on local roads and speeds 60 and below, most of the time ( 45 ) . as said before the 3x5 will destruct --48 states and highway speeds you might have to by 48 flags , that's a lot of beating back there
The flag on my avatar will be on a '80 FLT tomorrow , for 1 day hopefully it will last for next year

 

I've run 3x5 at highway speeds on my Sportster. Worst part is the shaking from the flags tends to destroy all my tail light bulbs, they just can't stand up to the shaking. I run two 5x7 flags for parades or slow speed escorts.