As far as I know, there are only 2 sizes of 'basic' Aspire
coils. (if we discount all the sub-ohm equipment). There are small BVCs and Nautilus BVCs. There were definitely small and large BDCs too. I believe the intention of the K1 was to build on the design and reliability of the mini Nautilus. They designed these
tanks to take the BVC coils.
However, the ETS and Vivi Novas originally came with BDC coils, so the physical size must mean you could use them on a K1. If you have some, try them with cheap
juice. Personally, I couldn't get them out of my clearomizers fast enough once the BVCs were introduced.