I have a broad knowledge of physics and engineering, but pressing/stamping is not my particular area of expertise.

The Wikipedia image linked by HankScorpi-vs-the-World indicates feature sizes almost as small as 100nm for Blu-ray (that's maybe a factor of 2- or 3 smaller than I was expecting, but may well be correct).

I suspect the problem of how small features can be pressed comes down to economics: I expect the metal stamping tool will wear with use, so that as the feature sizes get smaller probably you get get fewer good-quality stampings out of it before you need a new tool.

Although progress is being made in making LEDs deeper into the ultraviolet, given that once you get into the UVB range the light is strongly absorbed by plastic lenses and discs, I can't imagine this technology has a whole lot further to run anyway.