-During the '87 crash VIX was said to spike to >150, so let's assume a worst case of 200
-Third month futures are very unlikely to get anywhere near that high, so call a rough max of 100 (although it's probably considerably lower than that)
-That's ~600% higher than where things are trading now
-Select a survivable loss number from the position, say -30% on the account
-Then back into your max position size which would be contract value equal to a ~5% allocation in this example
Well, my position in VIX was -10 contracts, and about -40 contracts on VSTOXX. Taking VIX at 14, $1000 per point, that's $10,000 per point. My account size is about £550k - totally conceivable that it could have a one day jump of 20-30 points, which combined with likely falling equity indices (I trade NQ, HSI, SMI, Eurostoxx 50), could easily put me in margin call territory or worse.