A very quick Google search brought up this:
For large photo printing, Metro are the service leaders when it comes to supersizing your images for exhibitions, museums or the home. CPick from C Types, Giclée, Direct to Media or Vinyl and we'll produce award winning prints, just for you.
metroimaging.co.uk
They mention prints “on any substrate” up to ten by eight feet, which sounds like fence-sized. They offer drum scanning, too. I haven’t looked any further on the site, not even at prices.
It does seem sensible to begin at the result you want and work out what you need to achieve it, rather than the other way round.
My Epson 850 will resolve grain (on Foma 400) and any further increase seems pointless, although a friend remarked that it didn’t resolve the shape of the individual grains. He was quite right, of course. I have a dim recollection that very large prints use 240dpi, rather than the 360dpi of ordinary printers, which might influence your thinking. Makes sense, considering the likely viewing distance and the unwieldy file sizes.
There is also software which promises to improve fine detail, but I have no experience of using it.