A well-fitting suit looks better than an expensive one that fits poorly.
The jacket shoulders should end where your shoulders end, without divots or overhang. When buttoned, the jacket should lie flat without pulling into an X shape.
Sleeves should show about a quarter to half an inch of shirt cuff. The jacket length typically covers the seat of your trousers.
Trousers should sit comfortably at the waist without a belt being required, and have a slight break or no break at the shoe. A good tailor can adjust sleeves, waist, and hem, so focus on shoulder fit when buying.