
1024 bytes is equal to 1 kilobyte, which is itself equal to ¹⁄₁₀₂₄th of a megabyte. Internet speeds are usually measured in either some amount of bytes per second (KBps/MBps) or some amount of bits per second (kbps/mbps). (Like if you are testing for video call performance, as video calls transmit large amounts of data, or for gaming, as certain games send large packets with all the game data at once.) You probably want to just leave it at the default unless you have a reason to increase it. Some software also just isn't made to handle really large packets.

Sending large packets can use up your Internet connection's bandwidth and cause it to start dropping packets, leading to packet loss.

(Think of this as like the address on the top of a letter increasing the word count.) I give a range because how much metadata is needed to get them to the right place can make them slightly larger or smaller, and I can't guarantee the browser will always send them as a specific size. The packet size is how much data is in each "packet" of data that the test sends.
