wstrb
Burst size is the size of your data being sent, it can't exceed the databus size, but can be lesser than that.
Assuming your burst size is 32 and burst length is 4, it means, four 32-bit data will be sent as a burst.
Incase of AXI, if the burst size is specified as 16 on a data bus of 32, for write transactions, WSTRB is used to pick the bytes, which are part of the data.
e.g.
if data is FFCC, and if WSTRB is 1100, then, FF will be chosen based on the WSTRB value.
If anything is not clear, revert back