Akshar-Mala is an Indian poetic form in which the initial letters of each line are in alphabetical order. There are no other requirements for the form.
I found this form in The Dictionary of Indian Literature, One, Beginnnings-1850. I haven't been able to find any other descriptions of it, or any another Indian form that resembles it closely.
A while back we wrote in the form of iroha mojigusari, where the first letter of the alphabet starts off the first line and the second letter of the alphabet concludes it. The third letter starts the second line and the fourth letter finishes it. This continues until all the letters of the alphabet have been used in order.
Since there are no other guidelines here than order, I don't see why you can't begin with any letter of your choosing and make the poem as long as you like.
So, there's your challenge for the week. I hope you'll join me in writing an akshar-mala. Please share a link to your poem or the poem itself in the comments.