Well, many ppl here says that if you have a v1.2 or v1.3 that you should use a 246kb Bios because the tsop IS 256kB.
and that if you have a v1.0 or v1.1 Xbox that you should use a 1024kb Bios cause the Tsop for that version is 1024kb.
I personally have never used a Bios bigger than 256kb. and ive Flashed a lot of boxes.
You can use a 256kb Bios no matter what size your Tsop is (256kb or 1024kb). if the Tsop you are flashing is 1024kb and you use a 256kb Bios, Evox (use the latest) automatically mirrors the Bios into the 4 Banks. (verified)
this is of course if you are planning flashing the TSOP with only one (1) bios (ex. the whole TSOP with Executer 4977). if you want a multibios Tsop then thats another Story.
Hope it helps!!