It all depends on the bottle size and shape. The only true limitation you have is the size of the straight wall of the bottle (if a straight piece of paper encounters a curved surface it creates a wrinkle), everything else is personal preference. Put a ruler to the side of the bottle and measure how tall the straight portion of the bottle is (where the ruler's edge makes perfect contact with the glass). This defines your height limitation. Then depending on your design and personal preference, you need to decide if you want it to wrap around the back or be completely visible. If you're using a standard tall desert bottle, 3" x 5" works well. If it's the shorter Bordeaux shape, then 2.6" x 3.5" works well.
I almost exclusively make 3 gallon batches (except when I make kits), so I bottle in 375ml bottles mostly. A 3 gallon batch yields 24 375ml bottles (a case where I purchase them) and 3 750ml bottles. I have been using the OL150 label from onlinelabels and they are 3" X 4.33". They seem to work well on both the 375 ml Bordeaux bottles with the shoulders as well as with 750ml bottles.
Noontime is exactly right in what I have found. My wife insisted on buying some 375ml bottles which were extremely narrow and tall (Riesling style bottles), and I got the wrinkles he mentioned with the labels I use. For those, smaller (shorter) labels are needed. Those bottles have since been banned...lol