resolution - QGIS 2: How to create a regular vector grid at fine scale and for large area?

In QGIS, we wish to create a regular vector grid with 25*25m resolution and over an area of around 12000 km2. We tried to use: -vector>research tools> vector grid - and v.mkgrid function from GRASS via QGIS but this was not successful. We tried to create grid with smaller resolution and it did work. So, it seems that the issue is due to the amount of data that this procedure generates. if anyone has an idea about how to realize such a fine resolution grid it will be really much appreciated, thanks, H&M