。
They had built a fire big enough to roast a whole bull.
Literal
They [topic-は] bull [object-を] whole-roast do [about-くらい] [genitive-の] fire [object-を] were-making.
A vivid scene-setter, casual-rough in tone. やつら is a rough/casual plural of やつ ('guy, fellow'), often carrying a disdainful or distancing nuance — 'those guys,' 'those characters.' 丸焼きにする is the productive ~にする pattern applied to 丸焼き ('whole roasting'), meaning 'turn it into whole-roast form' = 'roast it whole.' The ~くらいの ('to the extent of / about') creates a comparative scale: 'a fire of a size such that you could roast a whole bull' — not that they were actually roasting one, just that the fire was that big. 火をおこす ('to start / build up a fire') is the standard collocation; the ~ていた (past progressive) describes the fire as already going when the speaker observed it.