"I'm sure he meant to use a smaller size on all (!) trades, not just after a drawdown."
Yes, obviously. The OP was trading too large on his trades in the first place, before this all happened to him. The only reason he didn't know it was because he was having success. Success often masks the problems lurking underneath. But if this drawdown has him asking other people what to do, that right there is proof positive that something went wrong that HE DID NOT EXPECT.
The answer is, trade smaller and get more occurrences. He should have been doing that in the first place, but the last thing you want to do when you have a bad system that didn't account for a large drawdown is to use the same damn shovel that got you into the hole to keep digging it.
The loss is LOST, can't "make it back." The best thing to do at this point is accept the loss is gone, and smarten up, tighten up the system, do better next time. With better trading going forward, he'll get back his high water mark and be a better trader in the future.
DO NOT get into the old, gotta make it back mentality. That's a very good way to double down on your losses.