These grow to massive proportions in the UK and today this article showing a massive Rhododendron that is almost unbelievable in size, but there it is, the 'largest' in England. Gardener prunes England's biggest rhododendron bush as it blooms just in time for end of lockdown | Daily Mail Online
Width-wise, I've never heard of anything like that. The tallest is seemingly in India, at around 108ft / 33m.
Wonderful specimen. Presumably the cultivar is 'Cornish Red'. Otherwise the one in India is an example of the wild species Rhododendron arboreum. I imagine it is located in a tree sided canyon where there is a combination of good soil, wind protection and side shading that has both enabled it to live long enough to get that tall without toppling yet induced it to stretch for better light exposure.
The one thing with Rhododendron's is that they grow very well here in the UK, but somtimes a little too well as we are all aware. I think if anybody has ever visited Exbury Gardens here in Hampshire, they will testify at their suitability to native soil.
I was a little confused at "The huge arboreum smithii, also known as the Old Cornish rhododendron species ...". I found a History of Rhododendrons page: JARS v44n4 - A History of Rhododendrons (vt.edu) that's like reading about old friends - almost everyone mentioned has a trail named for him at UBCBG. Anyway, it says
The Cornish Red is still being sold in Cornwall by local nurseries and online. They advertise it as getting to 80ft tall and 120ft wide at maturity. Now who on earth has a garden in the UK large enough to accommodate that. But I bet people with a postage stamp size garden still buy it !!!