Sandbags were 14" by 26" so yours seem to be modern larger ones.. more like mealy bags than sand bags
So how big would this translate to with 28mm figures? I just made them so they looked good with a space marine. Would smaller ones be preferable then?
Smaller ones only if you want to do historicals, and military.
mealy bags were used in south africa/ victorian times think a sack of maize, light for the size as its not sand.
modern non military use whatever sacks you can find.. old hessian potato sacks.. modern plastic sacks etc.
Historical ones were probably that size because when 14 by 26 and full I would imagine they would be narrower and shorter due the the 40lbs of sand filling the middle. Quite heavy to lift.
3 choices for an empty bag in historical sizes
true 25's or 1/60 = 6mm by 11mm
28's or 1/56 = 6.5mm by 12mm
33 or 1/48 = 7.5mm by 14mm
(all figure measurements to eyes and bag measurements rounded up to nearest half mm)
to be honest I think thats only slightly smaller than what you have done, looking at the photos.
look at the way sand bag walls were built too.

just found this for flood defenses "Do not use feed sacks, as they are too large to handle. Use bags about 14-18" wide, and 30-36" deep."
http://www.911sandbags.com/custom.aspx?id=3Good photo and lots more at
http://www.3-4cav.com/images/HqandHqTroop/6666660-R1-E052.jpgsandbag walls "should" be built with one or two rows two bags thick laid parrellel next to each other, the next row at right angles to a row below. So they should be as long as two bags are wide.
This "ties" them together so they are harder to knock down