Instruction: Extract information from the supporting document to answer the given question.

Input:
Question: What size of the post this monopod has? 1/4 or 3/8?
Document: While I have not used this monopod yet, I have extended it and examined it for quality, fit and steadyness. It passes on all 3 counts.
The Sirui monopod collapsed to the size I need yet it is strong, and stable. I will be on the lookout for more products from this brand.
I bought this monopod as something that is lightweight and easily packable in carry-on luggage. I'd been using a Manfrotto monopod which worked wonderfully, but is very heavy and does not break down to airline carry-on size.The Sirui is light-weight, easy to use and I love the clip that is attached for easy carrying.
I got this to use with my GoPro and could not be happier. The monopod is solid and can even be used as a trail trekking pole. I love how it collapses to a smaller size to easily fit in my pack. Combine this with the GoPro tripod quick release mount and you got a trekking pole with a camera, perfect for the trail.
Very sturdy, yet compact enough that it easily fits in the laptop compartment of my Lowepro Fastpack 250 along side my other gear when folded. Most other monopods this size only fold down to between 21.5 and 25 inches, this one folds down to 15. The only downside is that the compass is a cheap piece of crap, but the monopod itself is a very high quality product.
This monopod is sturdy , compact and works well. Even exerting a lot of downward pressure with a heavy set-up ( DSLR plus 70-400 mm lens) causes no bending or flexing with all sections extended. Adding a ball head will speed up the camera or lens mounting procedure ,however, at the expense of increasing the overall monopod length ; in my case its 3 " for a 6lb Velbon ball head .The Velbon Ultra Stick R 50 has about the same price , dimensions, load capacity and weight.
Purchased it as a video stick, very compact, well worth the cost for the size that I wanted, will buy it another one if I need to in the future.
I ordered this unit for two reasons: 1) when packing for my recent trip to Grand Teton and Yellowstone NPs, I had sufficient difficulty packing (for carry-on due to short connecting times) that I could not take my usual monopod, and 2) I saw the Sirui being used by a few other people with good success. It is not as light and is more expensive than my original monopod, but it is almost one-third shorter! It has a good quaility feel and a great finish.
One cannot go wrong with either monopod.
This is my first monopod and I love it. The ability to have it fit into my camera backpack is awsome. Quality product.
Answer: Both, it's reversable.