When I receive clients in my workshop, some parents want long dresses that will fall elegantly over their arm when the baby is being carried. Others, on the contrary, are afraid that the baby will be swaddled in a garment that is too long for them and will prefer to opt for short christening dresses. Don't forget that in between, you can ask for (for custom-made christening dresses) a mid-length dress that will fall very slightly below the baby's feet.
The length of the dress will also depend on the baby's age. I had a client who asked me for a long christening dress for her 18-month-old daughter. Obviously, at that age, a baby walks, and if you don't want to spend the entire ceremony carrying your child, who must weigh around 10-11 kg, I strongly advise against choosing a long dress. However, up to 1 year old, there is no problem.