The Old Folks are very particular about their crabs and have strict criteria on what crabs to choose and how they should be prepared.
First, the crabs must be alive and fresh, which is usually the case in Singapore and not a problem.
Second, the size should be around 1kg -- too small and it's not worth the effort; too large and the meat would be too stringy.
Third, whenever possible, choose pregnant female crabs for maximum roe, which unfortunately weren't available that day.
Fourth and finally, when cooking, do not, I repeat, do not, deep fry the crabs first. Most stalls do that to speed up the process, but it affects the final texture and meat sticking to the shell.