I found a partial answer to my problem in Zebra iMZ320 image printing feeds too much paper:
! U1 setvar "ezpl.media_type" "continuous"
! U1 setvar "zpl.label_length" "500"
but how to configure printer if, for every print job, the size (height) changes? for example, in my case, I need to print a timesheet report, and I want that the user indicates the number of rows to print every time. I must calculate and configure the size of printing area every time (from my android app), or is possible to say to printer "print everything between ^XA to ^XZ without feed blank paper" ?? thanks!