Just like with a dress shirt, the shoulder seam should sit on the shoulder bone. The sleeves should come down about half to two-thirds on your biceps. Anything longer looks vintage anything longer too trendy. Ideally, you want a very slim fit in the sleeve.
Polo Shirt - Front and back same lengthPolo Shirt with Long Tail Too Long
Traditional polo shirts are longer in the back than in the front and look better when tucked in. If tucked in, the shirt should not exceed halfway past your buttocks in the back or your fly in the front. When untucked, the polo shirt should sit just below the waistband / belt line.
Most polo shirts in the U.S. are sized the same as any other casual shirts in small, medium, large and extra large, etc. But for higher end brands or European brands you might also find suit sizing based on your chest measurement. In either case, it pays to either try things on or to consult the sizing table of each brand because a size L from one brand be a size M in another…