DEM (Digital Elevation Map) resolution in ArmA terrains.

90 meter SRTM is the best you are going to get, and it will work fine, especially if you use some erosion routines on the elevations. Using 50 meter cells in V3 is going to wash out most of the elevation detail anyway. I would recommend using a 10 meter cell, especially since I checked out the DEM for the area and the terrain is definitely not flat. Of course there is a performance trade-off in using a smaller cell.

Q: if the DEM/SRTM data is 90m… there is 80 wasted space if we make that for 10m cell size?

A: I wouldn't call it wasted space. The points on a smaller scale are interpolated and the results are usually good. Adding some noise and then doing erosion does a good job of bringing those intermediate cells to life.

10m DEM data is nice, but not that much different than using 90m+erosion (like Razani).

There is no way to create 100% accurate terrain for a great number of reasons. So striving to do the “best possible” based on accurate data is a worthy goal. Getting the general layout and “look and feel” will be more than enough!

Usually if no data can be obtained after querying Google a couple of times, chances are you're not going to strike gold. US NED data is available for free in:

1 arc second (~30m)

1/3 arc second (~10m)

1/9 arc second (~3m)

The few rare instances where DEM data higher than 3 Arc Second (~90 meter) res is available is usually provided by the government of the specific country.

Q: If not, can you use 90m DEM or is it just too low resolution?

A: I guess that would be subjective, personally I would aim to use the 90m data as a template and add in finer details utilizing 'artistic freedom (like Avgani for example).