The git command
git count-objects -v
will give you a good estimate of the git repository's size. Without the -v flag, it only tells you the size of your unpacked files. This command may not be in your $PATH, you may have to track it down (on Ubuntu I found it in /usr/lib/git-core/, for instance).
From the Git man-page:
-v, --verbose
In addition to the number of loose objects and disk space consumed, it reports the number of in-pack objects, number of packs, disk space consumed by those packs, and number of objects that can be removed by running git prune-packed.
Your output will look similar to the following:
count: 1910
size: 19764
in-pack: 41814
packs: 3
size-pack: 1066963
prune-packable: 1
garbage: 0
The line you're looking for is size-pack
. That is the size of all the packed commit objects, or the smallest possible size for the new cloned repository.