Are there two sizes for kick13.rom?
 

I have an original dump of my A500 kickstart rom from way back. 256kb. Everywhere I read on the net suggests it should be 256kb, but every pirated one I come across is always 512kb. It doesn't appear to filled with null data either, so what's the difference? TUDE I remember only worked with the original 256kb one.

Also, WinUAE accepts the 512kb fine.

The only 1.3 ROMs I have seen that are 512kb have been extracted from A3000s and contain two 256k Kickstarts one after another, why they are like this I really don't know though I have no doubt someone round here will.

No, all 1.3 machines will produce a 512kB image if you use transrom. The 256kB kickstart is visible twice in the address space.

If you really need a 256kB image and have a 512kB image, just split it in two.

Totally true what Jope said. In fact, they are commonly called overdumped roms and they add an "o" after their names.
But remeber it is a software issue that makes them twice their size. Real 1.3 ROMS are 256kb!