Have a look at dialog4ports(1), there are environment variables you can set. You could try D4PFULLSCREEN , or experiment with D4PHEIGHT, D4PWIDTH and the other variables.
Look also into dialog(1). You can create with the command dialog --create-rc .dialogrc a configuration file ( for root ) in which you can set various run-time configuration settings, in particular have a look at the --begin option:
--begin y x
Specify the position of the upper left corner of a dialog box on
I'm not entirely sure how to set it in ~/.dialogrc, try:
begin = y x
( replace y x with numbers, representing screen position values )