I have a 2 year old Chi/Long hair terrier mix, Hes only 8 lbs. Hes so small, friendly and loves to cuddle. They are a great (mix) breed if your looking into getting one. 
How Big Does A Chihuahua Terrier Mix Grow?
Really the dog would eb that big just a little bigger then a chihuahua.
Most terrier (mid sized terriors) ex: Carin terriors, west high land white terriors, scotties and what not mixed with a chihuahua lead to the normal small size of a chihuahua with atributes that are the grnetic strength in the breed: Ex: Westies the white soft over cote in her dog, carin chihuahua's usualy get the eyes of a carin and some of the spars scragaly hair, then the scotties(guessing) probubly pass on their well known black hair or maybe the spars long scragily hair of a carin. =)