Ubuntu 16.04 and barcode - how to set page size?

Question

Command line

barcode -p letter -umm -e 'code128' -b 12345678 | convert - /var/www/html/datamatrix/compile/img.png

Command example. A barcode is generated, but only in A4 format. The parameter -p letterspecifies a different size. Tried a numeric value like -p 215x279mm, but the result is the same.
Who can tell - maybe some libraries are missing (or any other software for ubuntu that generates code128).