The current maximum length is 2GB. However we would not recommend doing that for several reasons. Mostly because the activation process would take *forever*, and your app's performance would be terrible.
My recommendation is that if you're using more than a kilobyte for a field, then there are better ways to do what you want to do. Custom license fields are about "control flow" of you app (enabling/disabling certain features, allowing some customers to do certain things, etc.) and about storing meta-data.