xbox v1.0 : 3944, 4034 , 4036 , 4627
xbox v1.1 : 4817, 4972
xbox v1.2-1.5 :5101, 5530, 5713
xbox v1.6 : 5838
I would use 5713, however, like you said the one found on xbins is 1 meg. I am pretty sure with raincoat, you can still flash it as it will either resize it for you, overwrite itself 4 times, or just write the first 256k then the remainder will be lost in the void. Better get a second opinion on that.
If you want to resize it yourself there are many ways to do it, one would be to open it with " X2BM - XECUTER2 BIOS MANAGER v2.3" (if you can still find that app...hahahha) and just delete 3 banks and resave.
If you have a chipped box of the same version range, you can flash it to a free bank on that first to be sure that it works. As with any tsop flash, it is always good to try out an edited bios on something that you can easily recover from a bad/corrupt flash.
Good luck, I have done exactly that in the past but I used a resized bios, not the full 1 meg.