I agree, it depends on what age the pup is at that weight
If I remember correctly Prada was 5oz when was around a week old (when I saw the breeders advertisement). She is now almost 4lbs
With regards to taking care of little dogs, I can only say from experience that it really does make a difference. Our Chanel is very tiny (she is chubby and a ball of fur so she is now around 3lbs, but she is smaller than the smallest chihuahuas we have ever come across)
Comparing Chanel to my other 2, she has been a bit more hard work. She struggles to get up and down stairs (Unless they are very low). We have bought a lower couch so they don't hurt themselves when they jump off (and you have to keep an eye on that because they can be very fearless!)
But the BIGGEST scare was when I decided to give Chanel and my other 2 a piece of chicken breast as a treat when we were having a BBQ. Prada and Gucci ate it, but because Chanel is so small, the chicken got stuck in her throat (it was only around 2cm of meat no bones) and we nearly lost her, but thankfully a week at the hospital and she came back to us
I would definitely not recommend a very small dog as your first dog, but that may be just me
If you want the best seat in the house, you've got to move the Chihuahuas!