Hi Calvin,
4 to 4.5 inches long should be allright (for rifle up to 17 lbs).
Our regular counterweight (the small size, 1" dia, 4.7" long) is about 600 grams. It's approximately 21.2 oz.
Hope this help,
seb.
PS: Will send your goods tomorrow morning.
just order the weight and have seb put it in the box, or alternatively just order one from the usa distributor, nice guy and I have ordered stuff from him before.
and just wait until you see the new seb rest in the works,
I can hardly wait to try one this coming spring