In my past experience, the reservation agents almost always have taken a look at me (6'5" 265lb) and just booked me into a 2x seat whenever one is available. I don't know if they have some internal code or marking, but the other seat has stayed vacant, giving me a double to myself, as long as any open seats remain in the car. The only time I get a seatmate is when the car fills up completely.

This was more common back in the day when full trains were not so common. Nowadays, it might not work out that way. It also assumes that one reserves seats well in advance, as I usually do. When I have had to grab a train at the last minute, all bets are off and I get what is available, often with a seatmate. Even at my size, I don't usually have much trouble sitting with someone else in the ordinary car 2x seats.