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How big could an array of space telescopes acting as an interferometer be ?

How big would it have to be to resolve exoplanet surface detail the size of Iceland at a distance of 100 light years ?

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Well, with current or immediate-future technology, let's use LISA as an example of the largest baseline achievable at $5\times 10^6$ km. Then the diffraction limit says $\theta \sim \frac{\lambda}{D}$, where $\theta$ is angular resolution, in this case ($10^7$ cm)/($10^{20}$ cm)=$10^{-13}$ radians. At optical wavelengths, the LISA-type detector would have resolution of $10^{-16}$, so yes, in principle, this seems possible.

The biggest problem I think I am ignoring is flux: there wouldn't be enough photons coming from exo-Iceland to do proper interferometry, and then there is also the problem of contrast: what is the difference in flux between land and sea features, aside from contrast with host star.

With current radio telescopes, a much-quoted factoid is that you could read the date on a dime on the moon, but in practice you never could because it doesn't emit in the radio, and certainly not with that contrast in signal.

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