Tips have three numbers, the first number (5??) is the degree of the fan so 5 is 50 degree angle witch would give you a 10″ fan. 4 is 40 degree angle and would give you a 8″ fan. (If you are spraying at 10-12″ from the wall)
The second and third numbers (?17) are the orifice/size of the tip. the thicker the product the larger the orifice you will need. Make sure your machine can handle the tip size you require. Always check data sheets before you start spraying.
Most trade airless machines will come with a 517 tip from new. This is a good tip for emulsion, contract paints and can be used for smooth masonry. For products like acrylic eggshell 513 tip is a good size. Always make sure you are using the right gun filter for the tip and paint you are spraying.