This dissertation examines early-eighteenth-century understandings of literary length in order to shed new light on the structures of three long non-narrative poems of the period, James Thomson's 'The Seasons', Mark Akenside's 'The Pleasures of Imagination' and Edward Young's 'Night Thoughts'. Readings of these poems demonstrate the sophistication with which British eighteenth-century writers used extensive literary structures to represent, explicate and communicate objects and ideas that seemed too vast or complex for comprehensive description or narration. Part I of the dissertation surveys, in chronological order, earlier and contemporary critical theories which inform the three poems, in particular those found in the writings of two major Whig critics, John Dennis and Joseph Addison (discussed in Chapter 1) and in the poetry of Alexander Pope (Chapter 2). Considered collectively, these may be understood to describe a 'poetics of greatness' whereby extensive verse is progressively abstracted from its traditional generic loci and becomes associated more broadly with ambitions and potential failures of comprehensive representation and perception, with the sublime, and with playful or witty complexity. Part II covers the three long poems. Chapter 3 argues that in 'The Seasons' Thomson uses the figure of the maze to modulate allusively between stasis and motion, sublimity and playfulness, gesturing circumspectly towards a vast providential order. Chapter 4 offers close readings of two early Akenside poems and passages from Shaftesbury's 'Characteristics', a key source for 'The Pleasures of Imagination'. These reveal Akenside's abiding concern with the fine line distinguishing sublime inspiration from ridiculous delusion, which informs self-reflexively the very structure of his sublime long poem. In Chapter 5, perceptions of 'Night Thoughts' as too long provide the starting point for an account of how Young's belief in the didactic function of poetry translates into a temporal, cumulative poetics designed to wear its repetitive 'aperçus' on 'life, death and immortality', through the time of reading, into the heart of the reader. Just as in extensive classical genres like epic and georgic, these works invest structure with the task of transmitting an articulated experience or body of knowledge to the reader. As such, their parts are arranged coherently, if complexly, within the whole.