From the comment under my question ( may be someone could miss it, so posting it as an answer )[PS: slightly modifyed it]:

I think i found a solution how to change the width, but that's not the real solution.. I steel don't get it why thickbox width is 670px (as u can see in my code (will folow next) the global thickbox width is set like the passed arg!! ) So the solutins was:

function m7_resize_thickbox(){
  jQuery(document).find('#TB_window').width(TB_WIDTH).height(TB_HEIGHT).css('margi‌​n-left', - TB_WIDTH / 2);
}

jQuery( document ).on( 'ready', function() {
  tb_show( ed.getLang('m7.popup_title'), '#TB_inline?width=800&height=600&inlineId=mygallery-form' );
  m7_resize_thickbox(); // after tb_show we need to update width/height values; we could also do something like jQuery( window ).trigger( 'resize' );
}

jQuery(window).on( 'resize', m7_resize_thickbox );

From the comment under my question ( may be someone could miss it, so posting it as an answer )[PS: slightly modifyed it]:

I think i found a solution how to change the width, but that's not the real solution.. I still don't get it why thickbox width is 670px (as u can see in my code (will follow next) the global thickbox width is set like the passed arg!! ) So the solution was:

function m7_resize_thickbox(){
  jQuery(document).find('#TB_window').width(TB_WIDTH).height(TB_HEIGHT).css('margi‌​n-left', - TB_WIDTH / 2);
}

jQuery( document ).on( 'ready', function() {
  tb_show( ed.getLang('m7.popup_title'), '#TB_inline?width=800&height=600&inlineId=mygallery-form' );
  m7_resize_thickbox(); // after tb_show we need to update width/height values; we could also do something like jQuery( window ).trigger( 'resize' );
}

jQuery(window).on( 'resize', m7_resize_thickbox );