Take the last of the all business class nonstop flights to Singapore. The seats are the widest around. You won't have any problem at all. The seat measurement is between seats whereas in sleep position, you actually sleep diagonally and you put your feet into a cubby hole in the seat in front of you. Choose a bulkhead seat which may be better for you.

If you've never flown SQ, then go with them. You'd never want to step foot on another UA plane again.