Comfey is the shortest "fully evolved" Pokemon, at 0'04", though it isn't part of an evolutionary line. The shortest Pokemon that's part of an evolutionary line and is the final form would be a tie between Polteageist and Ribombee at 0'08".
The lightest "fully evolved" Pokemon is Kartana at .2 lbs., though, again, it's not technically fully evolved due to it not being in an evolutionary line. Cursola and Polteageist would be the lightest fully evolved Pokemon in an evolutionary line at .9 lbs.
Wishiwashi-Solo has the lowest BST of any "fully evolved" Pokemon at 175 BST, but you know what I'm going to say. As far as evolutionary lines go, Shedinja takes the cake at 236 BST.
The smallest and weakest? Wishiwashi-Solo is 0'08", .7 lbs., and has the lowest BST of a fully evolved Pokemon (though, again, it's not part of an evolutionary line). Shedinja is also only 2.6 lbs., though it's 2'07".
So I would have to give it to Wishiwashi-Solo since it's technically a fully evolved Pokemon.
Hope this helps! :)