Luxury cars come in compact, midsized, and fullsized sedans and coupes.
So it depends.
If the luxury car is a compact luxury car, then it is smaller than a midsized car.
Economy is a type of compact and subcompact car. They are fuel efficient and cheap cars.
Subcompact is smallest like Smart Fortwo, Toyota Yaris, Honda Fit, Nissan Versa, Chevy Aveo, Kia Rio, Hyundai Accent
Compact is Honda Civic, Toyota Corolla, Chevy Cobalt, Ford Focus, Mazda 3, Hyundai Elantra, Kia Spectra, Acura TSX, Lexus IS, Mercedes C class, BMW 3 series, Audi A4 or A3, Infiniti G35/37, etc.
Midsized is Honda Accord, Toyota Camry, Chevy Malibu, Ford Fusion, BMW 5 series, Audi A6, Mercedes E class, VW Passat, Infiniti M35/45 etc.
Fullsize is Ford Taurus or 500, Toyota Avalon, Lexus LS, Acura RL, BMW 7 series, Audi A8, Infiniti Q45, Chevy Impala, etc.
There are no luxury economy cars. but Lexus plans to make a Hybrid compact car, which will be a luxury economy compact car.