So, Coleridge's poem describes the layout of Kubla's palace as follows:
So twice five miles of fertile groundThe so-called 'Crewe Manuscript' contains slightly different measurements:
With walls and towers were girdled round:
And there were gardens bright with sinuous rills,
Where blossomed many an incense-bearing tree;
And here were forests ancient as the hills,
Enfolding sunny spots of greenery. (lines 6–11)
So twice six miles of fertile groundPurchas His Pilgrimes says:
With Walls and Towers were compass'd round.
In Xamdu did Cublai Can build a stately Pallace, encompassing sixteen miles of plaine ground with a wall, wherein are fertile Meddowes, pleasant Springs, delightfull streames, and all sorts of beasts of chase and game, and in the middest thereof a sumptuous house of pleasure, which may be moved from place to placeSixteen miles, not 'twice five' or 'twice six'. Why did Coleridge not write 'twice eight'? Impossible to know for sure, but it makes me wonder if reading Purchas happened between the writing of the poem and the writing of the preface, and the proximate inspiration for the poem wasn't a completely different book? Or maybe if Coleridge was reading other accounts of Xanadu at the same time? It seems unlikely I know: not only because the rhythm of 'In Xamdu did Cublai Can [build] a stately ...' seems so directly to inform the first line of Coleridge's poem, but also because of the use of the word 'fertile'. But bear with me. In Peter Heylyn's Cosmographie (1657) we read
the Great Chan's residence ... Xaindu the Royal Palace of the Emperour, of a foursquare figure, every side extending eight miles in length: within this Quadrant is another, whose sides are six miles long, and within that another of four miles square, which is the Palace it self, between those several Walls are Walks, Gardens, Orchards, Fish-ponds, places for all manner of exercise, and Parks, Forests, and Chases for all manner of Game ... and Careansu, near which there groweth an herb called Chiny-Cathay, of admirable effect against many Diseases; and so esteemed of by the Natives, that they value an ounce of this at a sack of Rhubarb. [Heylyn, Cosmographie (1657), 174]'Six miles' is one of the measurements in there, at any rate. Of course it's likely Heylyn derived this account from Purchas, and moreover that he mistakenly transcribed the latter's sixteen square mile plain as a square meauring eight miles along each side (which would be sixty-four square miles in total). But this account does have what Purchas doesn't: incense bearing trees. More, Purchas describes the Khan's palace as a kind of tent ('which may be moved from place to place'), where Heylyn appears to be describing a much more substantial and fixed structure, more akin to Coleridge's mighty dome. We do know that Coleridge read Heylyn (though he had a low opinion of him as a Christian: 'I scarcely know a more unamiable Churchman, as a writer, than, Dr. Heylin'), though I can't find specific evidence that he read this particular work. It was a famous and popular book, though, often reprinted.
I also don't know if Coleridge ever came across Giovanni Botero's Politia Regia (1620), and can't find any evidence that he did. At any rate, Botero himself is quite a famous figure, and his book includes the following:
et cum Cublai Cham ex illis cognovisset, illam civitatem rebellem futuram, curavit adificari aliam, cui nomen est Taindu, illi vicinam, qua in ambitu 24. miliaria continet, praeter suburbia: quodque in Palatio, quod in Xaindu habet, multi Astrologi & Nicromantici sint. [Giovanni Botero, Politia Regia: in qua totus imperiorum mundus eorum admiranda, census, aeraria, opes, vires, regimina et fundata stabilitataque (1718), 106. This is the later Latin edition of Botero's Della ragion di Stato, which he had completed in 1589. It is likely that Purchas derived his material on Xanadu from this book.]The Latin means: 'When Cublai Cham realised that the city planned, in the future, to rebel against him, he built another city, called Taindu, enclosing 24 miles within its circuit, including the suburbs, and in this palace, which is in Xaindu, are a great many astrologers and necromancers'. This suggests that Xaindu or Xanadu is the name of the province (Heylyn thinks it the name of the city; Purchas and Coleridge is ambiguous on the matter). According to Botero the name of the city itself is Taindu. That's quite interesting. Also, it seems the city is full of wizards. That could come in handy, in any future war Kubla Khan himself is planning ...