I'm affraid not, the available documentation is for the deluge API, and of-course general usage stuff, and not something like this unfortuently, where direct code-change needed as only solution.
Anyway, for just making the dropdown-paths bigger as per your pic and request, then you're correct and is on line '834' of '../deluge/ui/gtk3/glade/path_combo_chooser.ui' where you can up the number as per wanted, e.g. '299':
Code: Select all
<property name="fixed_width">127</property>