I'm looking to mess around with the calculation for hull HP with the intention of making it possible for ships to be badly battered but not blow up outside of a reactor/munitions explosion. I thought it was the init_design_specs file in the drawers folder but the only change I saw out of it was changing the hull of the transports and stations.

Related to this is the question of how is the size of a ship calculated?

I might be entirely wrong about this, but init_design_specs looks like the right place.

The hull value is defined on line 79 by the value "specs.hull".
And that in turn is defined on line 104 as depending on another separate (I think) calling of a hull variable which depends on the value "v.space", which I can't find reference to (in this file anyway).
It looks like that could depend on v.space being defined elsewhere.

Possible changing the last number on line 104 to something bigger might possibly do it without having to alter how the code works though.
specs.hull=round(hull_space_mult * hull / 10) * 10