How tall are the Posts on which you put the caps on?
According to the Field Manual, it appears that the low posts are 23 inches tall, and the high posts are 34 inches tall. I do encourage you to do a quick google search for these types of questions as it only took me about one minute.
Best of Luck.
Keep in mind you’ll actually have to reach higher because you have to bring the caps over the top of the posts and then drop them on. I’d reccomend 27" for low posts and 38" for high posts. Just to be safe.