Look at the SQ & UA seat maps (airline web sites or aerolopa)
UA Economy Plus is economy with more leg room. SQ PE is a higher class.
Look at the airline seat maps. Usually PE has less seats across the aircraft. so wider seats
Example (guess) -->
https://www.aerolopa.com/ua-boeing-777-gallery
United Premium Plus
Twenty-four Collins Aerospace MiQ seats configured 2-4-2
Seat width: 18.7"
Seat pitch: 38"
Recline: up to 6"
Aisle seats have moveable armrests
Adjustable leg rest and foot rest
13" HD touchscreen display
All seats are equipped with universal AC and USB sockets
United Economy Plus
Forty-six Collins Aspire seats configured 3-4-3
Seat width: 17.1"
Seat pitch: 34"
Recline: up to 4"
Aisle seats have moveable armrests except bulkhead rows
Adjustable leg rest and foot rest
9" HD touchscreen display
All seats are equipped with universal AC and USB sockets
United Economy
156 seats Collins Aspire seats configured 3-4-3
Seat width: 17.1"
Seat pitch: 31"
Recline: up to 3"
Aisle seats have moveable armrests
Adjustable leg rest and foot rest
9" HD touchscreen display
All seats are equipped with universal AC and USB sockets