Hi Tricia,
The original cell size would work fine. I changed it because the original single mesh had over 3 million cells, and I knew i would have to sub-divide it to get quick results. I find it easier to work with cells that have even, and equal, divisions, so I created cells that were 0.20 m a side.
That is an odd looking image, and does not resemble what I was seeing on your simulation. It looks like an underventilated fire. The simple mixing combustion model works by assuming that where ever fuel and oxygen mix in the stoichiometric ratio, combustion will occur. I think that is what you are looking at. The fire spread parameter refers to radially spreading fires over a burner surface, and is not what you are looking at.
The revised file I sent you has an instantly starting fire of ~ 6 MW, that appears to be coming from a mattress. That is not representative of a typical mattress fire.
Dave