Tough bags. Been riding around with me for a couple of years now.
How big...?? I'd guess 20-24" tall, 12" wide, 6-8" thick. Similar to a bag of cement.
If you're looking for race car ballast and don't go the secured weights or weight bar route, consider this suggetion:
A heavy anvil or vise in your spare tire well, covered by "spare" tire (the biggest tire that will fit in there on a heavy steel wheel...and fill that tire with water (7 lbs per gallon).
I say that as ballast that isnt't secured is illegal at NHRA and IHRA tracks for a good reason. If you go through tech with a "spare tire" (secured with the wing nut, of course) you shouldn't get harassed.
Off topic, how do you like your 4.7 for towing??