I tried it out setting everything to 100.0 but it seemed to have a cap on how much fire was allowed. ~10mx10m in one patch usually although I did manage to set a hillside (~50mx50m) on fire fighting the Cops/Army at one point.

Digging a bit deeper there are also fire burn times etc in 'explosionfx.dat' (for fires started by weapons I presume). @kaspervdh You might want to check out 'explosionfx.dat' also

& there is also a 'FIRE' value in 'gameconfig.xml':

<Item> <ResourceTypeName>FIRE</ResourceTypeName> <ExpectedMaximum value="32"/> </Item>

I messed with all three files for a while but couldn't get the fire to grow & spread across Los Santos as I had intended . I presume there is a hard cap on how much can be created but there may well have been something I've missed.