If you're not knowledgeable, have some one cut a couple of inches off your stabilizer. Might make all the difference and Hey, you don't need a 30" stab because the next guy has one.
No real determination as for length. More of personal preference. That said, the longer the stab the less weight it takes. Shorter stabs, say 24", need more weight to get the same effect. Okay, think leverage....
And the same runs true for use of back bars. Here's Griv's formula, just substitute length you have.
length of front bar times weight on front bar = "X"
Then:
"X" divided by length of back bar = weight on back bar.
example:
27" Front bar length times weight of 4 ounces = 108
108 / 12" rear bar length = 9 ounces for the rear bar.
You then take the 9 ounces, put it on the rear bar. If you do a true V bar, you split the weights between the two bars. If you do a side bar, you do it on the solo side bar.
You then add or remove weight on the rear bar only. Aim for the X. Remove or add weight until your side to side "misses" are down to a nice, ragged oval that basically kills the X.
So he's using 27" for the front and 12" for the back.
So, using 30" and 15";
30" times 6 ounces is 180 ounces
180 ounces / 15" is 12 ounces or 6 ounces on two back bars.