the date picker needs for self-sizing at least "selectedDate"
in its property array. So this is minimal and works:
put "" into tP["selectedDate"] -- yields "today"
popup widget "community.livecode.hermann.datepicker" \
at 100,100 with properties tP
if it is not empty then
put it & CR after msg
See the script of btn "POPUP" of the datePicker sample stack
for all options.
This is because sizing the widget requires script access which
is not allowed in OnCreate.
[You can also adjust width and height, use
put "220" into tP["height"]
put "220" into tP["width"]
but better use for that property pickerScale.]