could you help me to understand how the following requirement was computed:
Figure 2-1 and Table 2-7 walk you through the contents of the command word. Your confusion might be coming from the optional fields xCACHE and xUSER.
For a N Bit system without xCACHE/xUSER enabled when customizing the IP in vivado, the command word is N + 39 bits where N is the address width.
For a N Bit system with xCACHE/xUSER enabled, the command word is N + 47 bits.
And note that an AXI-Stream interface requires the data field to be a multiple of 8, so you may have to round up depending on your address width.
For a 64-bit address system without xCACHE/xUSER, 64+39 -> 103 bits which needs to be rounded up to 104.