I was using 1280x768 and it looked blurry. So I went to Displays and tried to change to the maximum resolution: 1366x768. But it threw a limit error. Only then I realized that was my old monitors resolution. How can I do it to use the maximum resolution of the current monitor and not the old one? This is what I see if I run xrandr:

alex@alex-K43U:~$ xrandr

Screen 0: minimum 320 x 200, current 1280 x 768, maximum 1366 x 1366

LVDS connected 1280x768+0+0 (normal left inverted right x axis y axis) 309mm x 174mm

   1366x768       60.0 +

   1360x768       60.0  

   1280x768       60.0* 

   1280x720       60.0  

   1024x768       60.0  

   1024x600       60.0  

   800x600        60.0  

   800x480        60.0  

   640x480        60.0  

DFP1 disconnected (normal left inverted right x axis y axis)

CRT1 connected 1280x768+0+0 (normal left inverted right x axis y axis) 338mm x 270mm

   1280x1024      60.0 +   76.0     75.0     72.0  

   1280x960       76.0     75.0     72.0     60.0  

   1152x864       75.0     60.0  

   1280x768       76.0     75.0*    72.0     60.0  

   1280x720       76.0     75.0     72.0     60.0  

   1024x768       75.0     70.1     60.0  

   1024x600       75.0     70.1     60.0  

   800x600        72.2     75.0     60.3     56.2  

   800x480        72.2     75.0     60.3     56.2  

   640x480        75.0     72.8     67.0     59.9  

   640x350        60.0     70.1  

alex@alex-K43U:~$ 

(My laptop's monitor is broken so I'm using an external mirror monitor)