I'm wanting to add a custom drop-down form field " Shirt Size" to a LP however, we currently do not have that field in our Marketo form field selection. How do I create the field without having to create a new field in Marketo that will syn to SFDC?
You can use a multipurpose Textarea field, like LastSurveyAnswers, for this purpose.
That field would be created in SFDC and allowed to sync as usual to Marketo.
Then you can change the label, input type, etc. on your form as needed. In JS or in a Smart Campaign, prepend the question(s) to the field value, so the full value appears like
T-Shirt Size: Men's Medium
then when that syncs over to SFDC, its meaning will be clear from the value, even though the field name will be generic.