When I create particle system I easly can control length and number of particles, but if I go to edit particles mode and brush some hairs - this option is disabling. Why? Is there a way to maintain the global settings while editing particles?
Short answer: no, you can’t. Changing these settings will recreate the hair from scratch, losing all editing in the process. If this is unacceptable, the only thing you can do is change the length and add hair with the tools in edit mode.