The size of halyard depends primarliy on two conditions. The height of your flagpole and the size halyard that your flagpole truck (pulley) will allow. This is also known as a sheave (pronounced  "shiv"). The sheave is the rotating, grooved wheel inside of the truck (pulley). This is the piece that the rope fits into.
 
The most common types of flagpole rope / halyard sizes are:
1/4"
5/16" 
3/8"
 
Our Flagpole rope stays firm and round over pulleys yet flexible with good elasticity.
Priced per foot
Solid Braided Multi-filament
Meets Federal Specification MIL-C-43307
Please specify number of feet you need
 
Pro Tip: When ordering rope for your flagpole, make sure you double the height of your flagpole (i.e., 40 feet of halyard for a 20 foot flagpole) and then add on 5' - 10' of extra rope to make sure that you are not short after you tie your knot and add on your snap hooks.