What is the meaning of Drill Size & Stop Mask errors in Cadsoft Eagle PCB layout ?
How can I fix it ?

 

Some components in the libraries have drill sizes that are below the minimums in DRC.

The minimum drill size is set in the DRC/Sizes dialog. I have mine set at 24 mils. If the drill size is too small, you won't be able to drill the holes without snapping lots of drill bits - if at all.

You can edit the library of the part that is showing the errors.

There is a ULP that will change pads in the library. Coincidentally, it's named:
change-pad-in-lbr.ulp

You can make mass changes using the ULP.
I suggest that you make a backup copy of the library before experimenting with that ULP. If you don't, and you mess up the library, you have no one to blame but yourself.

After you've updated the library, try adding another of the parts that had the drill size/stop size errors. Eagle will see there are differences, and ask if it's OK to update the part in the schematic. Let it update the part in the schematic, but don't actually add the part.

Then go back to your ERC and DRC to see if the errors have cleared up.

 

How can I change all the wire width on pcb layout at a time, without going to properties of each wire ?

 

1) Using the Group tool (looks like a dashed box, next to the wrench), select the area of the board that you wish to change the trace width.
2) Select Change (wrench) -> Width -> your desired trace width
3) Right-click on the PCB. Observe that all traces in the Group are now the new size.