Follow the guidelines in this topic to ensure that your file imports successfully.
You can import only one record type at a time. For ease of use, name the file the same name as the record type in Microsoft Dynamics CRM Online. For example, put all your contact records in a file named contacts.txt or contacts.csv.
You can easily create delimited .txt or .csv format files by using Microsoft Office Excel.
By default, the maximum size of the file that you can upload is 4 megabytes (MB). This can be changed by your administrator. More information: Microsoft Knowledge Base Article 295626.