Q. I was speaking to Rabbi Miller the other day on the phone (I live in Los Angeles) and he mentioned to me that pushing a stroller in a reshus harabim [public domain] is patur dorasia [not Biblicaly liable] because it is lemaleh myud tefachim [over ten palms] and no longer a reshus harabim. I would like to know if it is shayach [possible] for the Rav to ask Rabbi Miller the mekor [source] for that halacha as I am quite interested by such a sevarah, [reason], it would be a great tova [favor] for me!
A Gut Gebentchd Yur! Yasher Koach for the tremendous harbatzas Hatorah the whole year!

A. Chashukei Chemed (Shabbos 8a) discusses the proper way when in need (but not in danger of life) to take a child to a doctor or clinic on Shabbos where there is no eiruv. He suggests to wheel him in a children’s carriage that has three sides ten tefachim high (opinions vary – 80 cm or 100 cm), and an inner area of more than four by four tefachim, since in that case the carriage itself becomes a reshus hayachid or private domain. The Talmud (ibid.) teaches that when carrying or throwing a kaveres or bee hive with those dimensions, one is exempt from a Biblical prohibition. This added to pushing the carriage in an uncommon way (shinui), would create a double shebus or Rabbinical prohibition and thus permit moving the child.
He further recommends that the handles should be higher than ten tefachim, since above the height of ten tefachim, the reshus harabim turns into a makom p’tur or exempt area, and the carriage becomes attached to the makom petur. He quotes Pri Megadim (Eishel Avrrohom 266: 7) as a source.

Horav Shlomo Miller Shlit’a concords with the last ruling and the children’s carriage becomes agud bemokom p’tur when the carriage handle reaches over ten tefachim, thus removing the Biblical prohibition of carrying, but still prohibited Rabbinically.

Rabbi A. Bartfeld as Revised by Horav Shlomo Miller Shlit”a