Soul, you have hit the nail right on the head.
The timeframe you use MUST, in my unhumble opinion, relate to the stop placement strategy you use. For example, many of us use the previous high swing on a short. If that is too far away, we must pass on the trade so we need to choose a number that gives us logical exits that are comfortable to trade.
One approach is to look at the "average" volume of the market you trade. Just an eyeball calculation. Then, divide the volume into the approximate number of bars you want to see in a day.
I am a scalper and mostly trade the
DAX and Russel. Both now trade about 250,000 cars a day. 0.25% of 250,000 is 625 and gives me over 400 bars a day if I use a Volume Bar of between 315 and 625 (triand error eyeballing the charts for my setups) and get about 20 setups a day and a stop loss that fits my trading plan. If you do not want so many trades you can either have tighter criteria to maintain tight stops and a larger % of winners or you can slow things down by looking at 0.5% of 250,000. If you are a swing trader look at 1%.
Hope this helps. Great threads on
VSA and Volume candles.