While we had here some controvese discussions about sandbags, I did a Googlesearch and found several offers for sandbags.

https://www.amazon.de/20kg-30-60-10er-Pack/dp/B001GRRNMO/ref=pd_bxgy_229_img_2/261-7220417-1016717?_encoding=UTF8&pd_rd_i=B001GRRNMO&pd_rd_r=ed516aba-b2b1-42b4-8efe-c9059c55045d&pd_rd_w=ye2y7&pd_rd_wg=oOrQT&pf_rd_p=449f5fd6-8f81-46b7-aa57-ca96572671a1&pf_rd_r=EABKWDW7VQXNHMY5EX16&psc=1&refRID=EABKWDW7VQXNHMY5EX16

In a nutshell, the modern German Einheitssandsack is 300mm by 600mm or 1 foot by 2 feet and weight about 10 kg or 22 lb.
Not sure if these dimensions also apply to WWII or Vietnam sandbags.

Standard size for US Military sandbags is 14" x 26" (36cm x 66cm). Thay have been this standard size since before WWII.

Most modelers tend to do sand bags too fat. Plus you tend to fill sand bags with whatever is Handy to you. If your in the south Pacific, you'll use sand. Europe will mostly be clay. Vietnam is odd, as you might use sand if your on or near the beach, but more than likely you'll use red clay. Clay filled bags tend to be flatter, and is also the preferred filler. Over filled bags in walls tend to come apart at the wrong moment. You must tie the bags in together like laying bricks.
Why is dirt the preferred filler? Sandbags tend to catch all sorts of things flying thru the air. You rip a Chuck out of a sand bags filled with sand, and is soon empty! Clay is the opposite. A bag filled right will be about three inches thick. (75mm) walls always start wider at the base, and should lean in a few degrees. Color wise, I've seen grey, violet, and green, out of canvas. Yet I've used a basket weave green bag that appears to be made from nylon. It's not uncommon to see colors mixed, but keep in mind they tend to fade quickly. Lastly the woven bags are a semi gloss green, but not O.D. green.
Gary