With the sole purpose of commenting on my own experience.
Long ago I use the number for the fixed-length size of 208 for all visits, I remember I started with 180, but I found that some users Agent, originally created are really long.
I must confess that I tried to create a second limit for 19, (as you've mentioned), to raise an oscillation between 19 and 208. But I remember I could not create this second rule for 19 characters, in every attempt I was with error 500.
On the other hand, I'm not sure if there is a number, how length is safe to use. For these cases I try to be practical and realistic;
I can not satisfy everyone, and some of us humans are trying to make machines act more like humans.
One and two, for the days we live on the Internet, the security of my content became a matter of national security.
I have yet pending this second rule, one for length 208, and the second, 19 for length-short.