if your contour interval is 10 m u can easily make a 10 meter DEM and even better resolution

I am talking about best pixel size. I think 12.5 is the best calculating variogram. 

6,25m is the pixel resolution where individual pixels may just be distinguishable to those with excellent vision for the scale of 1/25000. The general rule you may use to calculate the pixel size is 0,25mm / scale.

An equation you can use that relates map scale (1/x), raster data resolution, and map display resolution is:

1/x = 1 / (pixel ground size [m/pixel] × pixel density [pixels/m]), so that 1 meter on the map represents x meters on the ground

http://blogs.esri.com/esri/arcgis/2009/12/04/mathematical-relationships-among-map-scale-raster-data-resolution-and-map-display-resolution/

Dear Ananta

I think the pixel resolution 12.5 is  adequate for calculating variogram.

Best regards

A good discussion on this is given by Tomislav Hengl: http://www.sciencedirect.com/science/article/pii/S0098300405002657

Thanks to all

Pixel size mostly related to your application, and to generate a correct DEM from that topomap u need to generate TIN by include all of points, lines a contours, then you can generate DEM with suitable pixel size of your application.