Finally, I solved it!! 
I use the jqxgrid inside a DIV wih jquery method .show(‘slow’)
First I load de grid and after that, when you press a button, the container div execute .show(‘slow’). jqxgrid has the size of the container div and, in the execution, jqxgrid reder again with the showing size.
Now i change ‘slow’ to ‘fast’ and put the code after this method.
Regards and many thanks.