If you are getting a male, I think you wont have to worry too much about it getting to 20ft. They average out at 12ft and exceptionally large males can occassionally reach 15-16ft.
Rabbits small up to extra large should suffice for a male burm for it's whole adult life, however in answer to your question, a local dealer to me used to feed his 19ft female on young pigs that he bought from a local abbatoir.
I think Goats are common for larger Burms, Condas and Retics too.
0.1 JCP "Zoot"
0.1 Greyband Kingsnake "Tansy"
0.1 Brazilian Rainbow Boa "Rio"
0.1 South Florida Kingsnake "Cromwell"
0.0.2 Crested Geckos "Duck and Drake (named by my 3 year old)"