You might try a 200% increase, which would take advantage of bicubic resampling. That then becomes your new native size ... sharpen appropriately then send that to the printer and let the driver create the final size.
This is how I interpreted what Jeff taught in Camera to Print, and have tried it a few times with good success.
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Yep...just about any decent capture (not so much a scan that has super high frequency film grain) can be upsampled 200% without too much effort. A really good capture may be able to go up to 400% if there is no camera shake but you usually must go through some more exotic processing routines. I've written about upsampling in this article: [a href=\"http://www.digitalphotopro.com/tech/the-art-of-the-up-res.html]The Art of the Up Rez[/url].
Sampling is performed before dithering, so the best thing to do is resampling to the driver PPI value.
While the nominal print head resolution is often stated as "360 DPI", that's a misnomer...in point of fact, the 78/9800 (including the 48 & 38 series) has a real head resolution of 180 nozzles/inch. The 11880 has 360 nozzles/inch. But the effective resolution actually comes from the stepper motors and the ability to make dots inline as the heads move back & forth.
If you don't care what the real print image size is, sure, you can resize without resampling to get a real 360PPI...actually, because of the Nyquist Theorem, it's actually useful to print out at 480PPI (beyond the real output resolution of the head). Note there are potential interference issues with Epson at certain higher resolutions.
However, it's wrong to characterize the Epson driver as doing "resampling" to make it's dots...it doesn't resample. The error diffusion halftoning is far more complex and exotic. As a result, you really DON'T gain any real advantage to actually resample images merely to get to some magic number. In the case of downsampling you are throwing useful pixels away and in the case of upsampling, you are making up pixels out of essentially nothing (just the surrounding pixels).
So, if the image size you want is 11 x 14 exact, and the resulting resolution is 269 pixels/inch, print it at 269 PPI–don't resample it to get it to 240 or 360. You are either throwing pixels away or making them up...
As for the "native resolution" of your image...you'll always have an actual pixel dimension of x number of pixels by x number of pixels. Photoshop only applies a size and PPI as a metadata tag stating the ratio (x" X x" @ X PPI). The main point is if the PPI is between 180 & 480 PPI, DON'T resample merely to get some exact PPI for the driver. It's simply not worth it. You would be far better off using your REAL PIXELS and sharpening for the final PPI (that's what output sharpening is all about).
It also means you DON'T need to keep different iterations of a file laying around with different sizes and resolutions as you can store the sharpening in layers that you turn off/on as you print different sizes. It's the basis of the concept of using Lightroom as a print engine...and when LR gets better output sharpening, you may not even have to go into Photoshop for final output sharpening!
If you _DO_ need to upsample, then do it bigger than you "need" and apply all the post upsample processing that gives you a good upsampled output.
