Typically, the answer is "maybe." If literally no one has ever documented such a sandbag AND no one has ever used such a sandbag even if not documented, then it might be novel (e.g., not anticipated). But even then, it could be obvious. For example, perhaps a foam form of those dimensions was known, and a person of ordinary skill would have known that a foam form could be replaced with with a sandbag form. Or perhaps you're addressing a well-known problem that demands a sandbag as a solution, and the only sandbag that could possibly work is one with the dimensions and properties you describe. There are a lot of ways something is unpatentable even if you can't find that specific thing disclosed in the literature.
Also, if you are trying to patent a system in the United States, the purpose of that system might not be distinguish your invention from prior art. For example, pencils are known in the art, so you're unlikely to be granted a patent on an application for "A system comprising a pencil wherein the pencil is used to [do something no one has ever done with a pencil]" Your claim is still to a pencil.
Almost finally: what "should" happen or "might" happen or is "likely" or "unlikely" to happen is different from what will happen -- the patent attorney, luck, and the examiner all affect whether an application is allowed as a patent. And even if it allowed, whether that patent is of any commercial or litigation value is a different question.
Finally: I'm not your lawyer and not giving you legal advice.
