youonlylivetwice,
You have (sort of) asked about two subjects.
Safety is something that you have to remember every minute. Starting with launcher setup, be sure to keep everyone back from the launcher by the recommended distances. This is to help protect you (primarily) from any fragments from a failed motor, but also from flying bits of debris that can be spit from the nozzle and bounced off the blast deflector. You get a better view anyway from slightly further away.
As the rocket launches, everyone should be paying 100% attention to where it goes through the entire flight. You need to make sure it goes upward and well away from anyone on the ground (or loudly and quickly call out a warning), that it keeps going steadily and smoothly upward (if it looks unstable, call out a loud warning), that it properly deploys a recovery system at the peak (or if it fails to deploy, call out a loud warning and point to the rocket so anyone not tracking it can get a clue where the danger is), and to watch where the rocket heads/lands so you have some idea where to find it.
After it launches, you also need to put a cap on the top tip of the launch rod. There can be many people excited and running around, and it is very easy to lose track of where the launch rod is standing, and to impale yourself on that "invisible" tip. Use whatever is handy; I like an old stale tennis ball, or to place the launcher high enough (like on top of a table) to the tip of the launch rod is well above eyeball level.
When you see where the rocket is landing, be careful about running to catch it or pick it up. Running itself is OK by me (silly as it may sound, this has been the subject of much discussion in the past) but you still have to pay attention where you are going, what you are about to run into, and (if you are crossing a street) what vehicles might pose a danger. Just use common sense. Chasing (or catching) a $10 rocket is not worth a hospital visit.
Warn everyone ahead of time that even several minutes after the rocket lands, the rocket motor itself can still be very hot.
What many of us do when setting up for a launch in a tight launch field is to use an old "clunker" on a small motor for the first launch. From there you can see which way the winds aloft are blowing, and how fast, and how you might need to adjust the tilt of your launcher. So after one of your babies has a fin cracked off, or some other damage, just patch it up and keep it for your weather-check rocket.
Remember that you can get more rapid descent while still being safe if you use simple "nose-blow" recovery without any streamer or parachute. If the nose cone is tethered to the rest of the rocket by a shock cord (or kevlar tether line) it will tumble down a bit more rapidly. If you are launching over a grassy area, you should have minimal landing damage and should still be OK.
Field size is always a bit of a booger. Of course, bigger is better here. It could be worth your trouble to drive a few minutes to someplace with a bigger field, like a school, or maybe an undeveloped piece of land. The range sizes shown in the safety code are MINIMUM, and I join with Schuyler in putting a big recommendation for finding a bigger range, if possible.
You asked "what is a safe height for a given field size?" and I would point to the same field size table that is listed in the safety information. Field size increases in proportion to motor impulse, and probably assumes some correlation between motor impulse/size and attained altitude (as well as relatively low winds).
Are you familiar with the A, B, C etcetera classifications for motor impulse, and what that means in terms of thrust levels, thrust duration, and general motor "power"?
