What is burst length with respect to the AXI specification?
If you are reading the spec you will see it says that burst length is the number of data transfers per burst, which they call beats.  Each beat can be a number of bytes specified by burst size.

So for example if you wanted to transfer 8 bytes starting at address zero you could use a burst size of 1 byte, and a burst length of 8.   Then you would get 8 sequential transfers of 1 byte for a total of 8bytes.  It's up to the slave to increment the address to the next byte location and put it out on the bus.