How Big Is A Full Grown Pocket Beagle?

There is really no such thing as a 'pocket' Beagle so there is no answer as to size. It's another one of those made up sales terms like teacup. Use of the term would be considered a huge red flag. It is likely either the dog will fall into or be close to normal size ranges for a Beagle or there is likely to be additional health issues due to or resulting in a dog being so undersized (like maybe a liver shunt) or being bred for it. Here is the breed parent club and if you are looking for a smaller Beagle you may also check the rescues and petfinder and responsible breeder's may have a pup that is likely to be smaller than normal (they will not use terms like that) but it isn't recommended to purchase one from someone breeding for it and using those terms.
clubs.akc.org www.petfinder.com www.wildheirlabradorrescue.org
www.dogfoodanalysis.com