As mentioned before, this form is not unique to Vishnu or the Avatars of Vishnu, but also other deities like Shiva as the Maha-Sadashiva and Ganesha as the Maha-Ganesha have been attributed with having their own Universal forms. The same is true in Shakti worship or the worship of the Divne Mother or the Mother Goddess.
The Mantra given in this post is the Vishwaroopa Mantra of Shri Ganesha, but it can also be easily chanted for any other deity, depending upon the religious or spiritual leanings of the individual.
Om Vishwaroopaaya Namah ||
ॐ विश्वरूपाय नमः ||
Meaning- Every being in the Universe is a form of Ganesha, I pray that Ganesha.
This Ganesha Mantra is said to give results to the devote and spiritually elevate him to a different spiritual level after the completion of 800,000 Mantra Chants. The Mantra Chants should not be counted and the Mantra should be practiced as a long term Meditation.