The depth of field has NOTHING to do with the sensor size
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DOF and OOF effects are however deeply related to the size of the image that you form on the sensor.
If you photograph the same subject from the same distance and want an image on the sensor of twice the (linear) size in order to make full use of a larger sensor, then you need to use twice the focal length, and then with the same aperture ratio, the circles of confusion at each point of the image will have _four_ times the diameter.
Thus when you present the same sized image of the subject on the print or on-screen, so using half the degree of enlargement, the COC at each OOF point on the print will have twice the diameter. That is half the DOF and stronger OOF blurring by any measure.
Alternatively, if you use the same effective aperture diameter (entrance pupil diameter) and thus twice the aperture ratio with that doubled focal length and image size, each COC has twice the diameter, and by the way each diffraction spot has twice the Airy disc diameter.
Thus when you display at equal image size, all COC's and diffraction spots on the displayed image are of equal size: exactly the same DOF, OOF blurring and diffraction blurring.
Thus 15MP in any format is equally useful for landscape photography so long as diffraction control does not force you to use f-stops so low that lens aberrations impair image quality. You just have to choose a different f-stop (same effective aperture diameter) to get a given combination of OOF and diffraction effects when filling a different frame size with the desired image.
With good SLR lenses giving best resolution at f/5.6 or even f/4 or below, diffraction/aberration trade-offs still allow resolution well beyond what any curent DLSR pixel size offers. Three micron pixel spacing might start pushing the limits of the highest resolving SLR lenses, but that would be 25MP in 4/3, 37MP in EF-S, etc.
P. S. NikonsR summarizes my point very nicely in a different way, particularly in his final paragraph.
Diffraction like lens aberrations lead only to a case of
the law of diminishing returns for the increases in overall resolution given by increasing sensor resolution; never a loss, and so far we are not at the point beyond which there is no significant improvement with further sensor resolution increase, so long as good lenses are used.