Hey Jennifer Borelli,
In setting up user permissions, you can give someone the ability to import lists or not, but cannot put restrictions on the types of lists they can import (see: Descriptions of Role Permissions - Marketo Docs - Product Docs).
If you're concerned enough to want to restrict the size of the lists users can import, I'd say you're probably better off restricting the import permissions to a few key people who have been well trained on the process.
Hope that helps.
Like Grace says, in the UI there's no restriction.
If you have a developer, though, you can build a substitute API-based upload interface that validates the size of a list before uploading it (and validates anything else about it, too, if they want to get elaborate). Not saying this is an overnight project, but it's easy enough if you have someone technical on the team.