Regarding the "how big should my log file be permanently set to after I back up and shrink it?" question:
I came across a very nice script
that will tell you when the most recent database or log autogrow events occurred. You could add this to a daily routine server check Agent job.
It's been useful to me as an alert that a log file blew up b/c of some extraordinary activity, at which point I could decide to (a) do nothing, b/c it will happen again in the future and the new size is OK or (b) take remedial measures.
If you do that, it might give you some piece of mind that, if you guess too small for your log file, you'll at least be notified that an auto-grow event took place. You generally want to avoid those from happening: explicitly set the size of your log big enough so you don't have them.