can i boot from a blue ray? [duplicate]

It's just a different type of medium. As long as you burn bootable ISO on it and boot it from drive which is seen by UEFI/BIOS it should work. I have several bootable BD's created using dd in Linux and/or Acronis in Windows and they normally work as expected.

As for size question, that's interesting. I believe ISO 9660 can have 2^32 blocks which is 8TiB and that's far away from current BD size ~100GB. So as long as booting is implemented properly according to ISO 9660 standard you should be theoretically able to use whole BD size. There might be problem with file size (that depends on ISO Level of interchange - Level 1,2,3 thing...) and with loaded software itself which might not be able to work with such large volume. Also not all tools for burning support unlimited (read maximum) ISO 9660 volume size.