If you can see a golf ball out to about 230 yards, you will have absolutely no problem seeing that trainer out to 400 yards, and that's about as far as I let my students get from us. The field is a lot bigger, but I see no reason to use it. Geez, that's almost a quarter mile.

All kidding aside, if you have an instructor, ask to stay within your visual limits, and let the instructor know when you're reaching those limits. Clear communication is essential, and it's a two way street. Get your instructor's evaluation, and give him any information that helps - like your visual limits, things that push your comfort level, etc.

Good luck!
Dave Olson