When creating a new record, I need to be able to show a popup with some text. I tried using the help text on my custom fields but the field is too limited in size.
What are the alternatives?
How I understand this question: You want to be able to show some popup with text providing an explanation about the field or something on the page. You tried using the help text on a field but got stuck on the help text field limitation. (size etc)
What could be a solution for you is to use the context-sensitive help setting of your custom object. This is the page that you see when clicking on the "Help for this Page" for that specific object. You also get this link when you are in the form to create a new record of your custom object.
You can create such a custom contextual help page by creating a new Visualforce Page. You don't need any code for it though, the visualforce page can just be plain an simple HTML / formatting.
You can set this visualforce page as the contextual help by going to your custom object (create > object menu), editing the custom object setting and there you will have a field "Context-Sensitive Help Setting". Point that field to your newly created visualforce page.