Input Mask: Is it possible to validate the entry length dynamically?

It can be very useful if we have a way to allow different character totals. For example, when I tried to use it for my Twitter account URL's. But they can be a carry of lengths after the twitter.com portion. Is this doable?

Unfortunately, it's not possible to dynamically validate the entry length, but I would recommend you to try with the Short Text Entry field validation instead. 

Adding the field to your form: 

Then open the field settings and go to the options tab and select "URL" as validation:

Here is an example about how that would work: https://form.jotformpro.com/72296222856967 

I hope this helps.