I look for options, but mostly good 'Flow'

I want the trail to make me work, hard. As In doing it right pays off with a good thrill of accomplishment and I want to feel like I had a workout but not so hard that I have to stop and puke at the trails end
I tend to stay off most black diamond trails as the potential for Injury Is just too much for my tastes, I have to go to work and pay bills.
The few Black Diamond trails I ride have bail outs or hard section bypasses that I can use, some are aerobically too much for me, bail outs and bypasses let me 'Work Out' on them.

The 'Blue' Intermediate trails I ride are quite a challenge and If I want more I turn up the speed some.

My single most Important tip/advice:

Water run off !
Trails must not become washouts or rivers, water will destroy a trail.
Always engineer a trail to shed water. If possible water should move down a hill away from the trail and when crossing a trail It should cross in a wide as possible path..
Deeper fast water cuts deep, shallow wide slower flows move less hard pac....
Low maintenance Is a big deal..

As for minimum length~ We have a few short trails of 100-200 yards or so, they are great because of the Intensity of the features.
Gator pit:
A 15' drop and a 15' climb, Can be ridden with very little pedaling if you 'Drop In fast'.
Followed by a hair pin left ending in a two foot straight down air drop onto a down slope, At the gate to the drop are two close trees...
Then a speed building run with a few small jumps all the while arcing right and getting sandy...
The challenge here is to hold your speed as the sand gets deeper because the climb out is mean.
At the top of the climb out is a switch back and the last leg Is a much needed flat rest run to the end with an area to re group before entering Golf Cart, a much longer trail.
The 'Fun half' of Gator pit,,I think most run It In less than 30 seconds, I never pass it up

SWAMP Mountain Bike Club Trails in Florida

SWAMP Balm Boyette Scrub Preserve Mountain Bike Trail