I tried reproducing the scenario you describe, but it seems to work as you intended, without showing any odd sizing for the lower dialog box.
Here is a card with my reproduction that you can paste into a the Dialogizer deck to compare to your own:
%%CRD0{"c":{"name":"card1","script":"s_lower:() \ns_lower.pos:\"dialog_lower\" \ns_lower.size:300,0\ns_upper:() \ns_upper.pos:\"dialog_upper\"\ns_upper.size:200,0\n\non command x do \n if x~\"lower\" \n dd.style[s_lower] \n elseif x~\"upper\" \n dd.style[s_upper] \n end \nend","widgets":{"button1":{"type":"button","size":[60,20],"pos":[23,221],"script":"on click do\n dialog_drip.view:\"none\" \n dd.open[deck] \n dd.say[text_a.value] \n dd.close[]\nend","text":"Do It"},"text_a":{"type":"field","size":[150,116],"pos":[23,92],"value":"!upper\n\nthe upper text but long enough to demonstrate that it's wrapping and so on and so forth, blah blah blah\n\n!lower\n\nthe lower text. again, this needs to be appropriately long to demonstrate text wrapping."},"dialog_lower":{"type":"button","size":[60,20],"pos":[133,294]},"dialog_upper":{"type":"button","size":[60,20],"pos":[236,43]}}},"d":{}}
Please make sure you're using Decker v1.66 (see Decker -> About in the menu) and Dialogizer v1.8 (see File -> Resources in the menu and select the "dd" module); using an outdated version of Decker with a modern Dialogizer or vice versa might cause strange behavior?