Open as large as possible. Keep all the information then sharpen as needed.
If you need to scale the image to smaller dimensions, do that in Photoshop using BicubicSharper.
Or wait here a bit longer. Lots of people here have opinions.
ACR opens the raw file at its native pixel dimensions, for the E-510 2736x3648. The different resolution choices are for saving out of ACR or into Photoshop. There is no advantage to saving or opening other than at the native resolution.
There is no advantage to saving or opening other than at the native resolution.
Actually, not entirely true. In the case of Fuji DSLR's you would do better to double the rez in Camera Raw as that matches the interpolation that the Fuji software does in their higher quoted "effective resolutions"(it ain't real resolution mind you but it can benefit certain image types).
If you know for an absolute fact you need more resolution than the native file has, you really might want to test upsampling in Camera Raw as it has a newly tuned upsample (put in in ACR 5.2) that is an adaptive Bicubic algorithm that is either the normal Bicubic or Bicubic Smoother depending on the size–and that's something even Photoshop can't do.
But in general unless you know for a fact you need the image bigger (or have a Fuji camera) processing at the file's native size is the most efficient.